El código Xaml

<Grid
    x:Class="Splash_Screen.ExtendedSplash"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="using:Splash_Screen"
    mc:Ignorable="d"
    Background="#185F18">

    <!-- la imagen tiene 620 x 300 -->
    <Canvas MaxHeight="300" MaxWidth="620">
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="35" />
                <RowDefinition Height="225" />
                <RowDefinition Height="40" />
            </Grid.RowDefinitions>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="60"/>
                <ColumnDefinition Width="140"/>
                <ColumnDefinition Width="420"/>
            </Grid.ColumnDefinitions>

            <Image x:Name="extendedSplashImage" Grid.Row="0" Grid.Column="0"
                   Grid.RowSpan="4" Grid.ColumnSpan="4" 
                   Margin="0" Source="///Assets/SplashScreen.png" 
                   ImageOpened="extendedSplashImage_ImageOpened"/>
            <TextBlock x:Name="txtTitulo" x:Uid="txtTitle" 
                       Grid.Row="0" Grid.Column="0" Grid.ColumnSpan="3"
                       Foreground="White" FontSize="18" FontWeight="Bold" 
                       HorizontalAlignment="Center"
                       Text="Prueba de Splash Screen personalizada" />
            <TextBlock x:Name="txtCopyr" Grid.Row="2" Grid.Column="2" 
                       Foreground="White" FontSize="15" FontWeight="Bold" 
                       Margin="0,10,0,0"
                       Text="©Guillermo Som (elGuille), 2013" />
            <Image Grid.Row="2" Grid.Column="0" Grid.RowSpan="2"
                   Height="30" Width="30" Stretch="Uniform" 
                   Source="///Assets/SmallLogo.png" Margin="0,0,0,10"  Opacity="100" />
        </Grid>
    </Canvas>
</Grid> 

El código para Visual Basic .NET

'------------------------------------------------------------------------------
' ExtendedSplash                                                    (19/Ene/13)
' Usar un SplashScreen personalizado
'
' Basado en un ejemplo de la MSDN (y creo que mejorado y simplificado)
' Sólo hay que modificar App.OnLaunched
'
' Cómo extender la pantalla de presentación
' http://msdn.microsoft.com/es-es/library/windows/apps/xaml/hh868191.aspx
' y Directrices y lista de comprobación para pantallas de presentación para
' Evitar un parpadeo durante la transición a la pantalla de presentación extendida
' http://msdn.microsoft.com/es-es/library/windows/apps/hh465338.aspx#ts_flicker_cs
'
'
' ©Guillermo 'guille' Som, 2013
'------------------------------------------------------------------------------

Imports System
Imports Windows.ApplicationModel.Activation
Imports Windows.Foundation
Imports Windows.UI.Core
Imports Windows.UI.Xaml
Imports Windows.UI.Xaml.Controls

Partial Class ExtendedSplash
    ' Rect to store splash screen image coordinates. 
    Friend splashImageRect As Rect
    ' Variable to track splash screen dismissal status. 
    Friend dismissed As Boolean = False
    ' Variable to hold the splash screen object. 
    Private splash As SplashScreen

    Friend rootFrame As Frame


    Private showWindowTimer As DispatcherTimer
    Private showWindowTimerNum As Integer = 0

    Private Sub OnShowWindowTimer(sender As Object, e As Object)
        showWindowTimerNum += 1

        If showWindowTimerNum = 1 Then
            ' Activate/show the window, now that the splash image has rendered
            Window.Current.Activate()

            ' aquí hacemos un pequeño descanso antes de mostrar
            ' la página principal
        ElseIf showWindowTimerNum >= 50 Then
            showWindowTimer.Stop()
            cargarMainPage()
        End If

    End Sub

    Private Sub extendedSplashImage_ImageOpened(sender As Object, e As RoutedEventArgs)
        ' ImageOpened means the file has been read, but the image hasn't been painted yet.
        ' Start a short timer to give the image a chance to render, before showing the window
        ' and starting the animation.
        showWindowTimer = New DispatcherTimer()
        showWindowTimer.Interval = TimeSpan.FromMilliseconds(50)
        AddHandler showWindowTimer.Tick, AddressOf OnShowWindowTimer
        showWindowTimer.Start()

    End Sub

    Public Sub New()

        ' This call is required by the designer.
        InitializeComponent()

        ' Add any initialization after the InitializeComponent() call.

    End Sub

    ''' <summary> 
    ''' Constructor with splash screen information 
    ''' </summary> 
    Public Sub New(splashscreen As SplashScreen, loadState As Boolean)
        InitializeComponent()

        If DateTime.Now.Year > 2013 Then
            txtCopyr.Text = "©Guillermo Som (elGuille), 2013-" & DateTime.Now.Year.ToString
        End If

        ' Listen for window resize events to reposition the extended splash screen image accordingly. 
        ' This is important to ensure that the extended splash screen is formatted properly in response to
        ' snapping, unsnapping, rotation, etc... 
        AddHandler Window.Current.SizeChanged, AddressOf ExtendedSplash_OnResize

        splash = splashscreen

        If splash IsNot Nothing Then
            ' Register an event handler to be executed when the splash screen has been dismissed. 
            AddHandler splash.Dismissed, AddressOf DismissedEventHandler

            ' Retrieve the window coordinates of the splash screen image. 
            splashImageRect = splash.ImageLocation
            PositionImage()
        End If

        ' Create a Frame to act as the navigation context  
        rootFrame = New Frame()

        '' Restore the saved session state if necessary 
        'RestoreStateAsync(loadState)


    End Sub

    'Public Async Sub RestoreStateAsync(loadState As Boolean)
    '    If loadState Then
    '        Await SuspensionManager.RestoreAsync()
    '    End If
    '    ' Normally you should start the time consuming task asynchronously here and  
    '    ' dismiss the extended splash screen in the completed handler of that task 
    '    ' This sample dismisses extended splash screen in the handler for "Learn More" button for demonstration 
    'End Sub

    ' Position the extended splash screen image in the same location as the system splash screen image. 
    Private Sub PositionImage()
        extendedSplashImage.SetValue(Canvas.LeftProperty, splashImageRect.X)
        extendedSplashImage.SetValue(Canvas.TopProperty, splashImageRect.Y)

        extendedSplashImage.Height = splashImageRect.Height
        extendedSplashImage.Width = splashImageRect.Width
    End Sub

    Private Sub ExtendedSplash_OnResize(sender As Object, e As WindowSizeChangedEventArgs)
        ' Safely update the extended splash screen image coordinates.
        ' This function will be fired in response to snapping, unsnapping, rotation, etc... 
        If splash IsNot Nothing Then
            ' Update the coordinates of the splash screen image. 
            splashImageRect = splash.ImageLocation
            PositionImage()
        End If
    End Sub

    Private Sub cargarMainPage()
        ' Navigate to MainPage 
        rootFrame.Navigate(GetType(MainPage))

        '' Set extended splash info on Main Page 
        'DirectCast(rootFrame.Content, MainPage).SetExtendedSplashInfo(splashImageRect, dismissed)

        ' Place the frame in the currrent window 
        Window.Current.Content = rootFrame

    End Sub

    ' Include code to be executed when the system has transitioned from the splash screen to the extended splash screen (application's first view). 
    Private Sub DismissedEventHandler(sender As SplashScreen, e As Object)
        dismissed = True

        ' Navigate away from the app's extended splash screen after completing setup operations here... 
        ' This sample navigates away from the extended splash screen when the "Learn More" button is clicked. 
    End Sub
End Class

El código para C#

//-----------------------------------------------------------------------------
// ExtendedSplash                                                   (19/Ene/13)
// Usar un SplashScreen personalizado
//
// Basado en un ejemplo de la MSDN (y creo que mejorado y simplificado)
// Sólo hay que modificar App.OnLaunched
//
// Cómo extender la pantalla de presentación
// http://msdn.microsoft.com/es-es/library/windows/apps/xaml/hh868191.aspx
// y Directrices y lista de comprobación para pantallas de presentación para
// Evitar un parpadeo durante la transición a la pantalla de presentación extendida
// http://msdn.microsoft.com/es-es/library/windows/apps/hh465338.aspx#ts_flicker_cs
//
//
// ©Guillermo 'guille' Som, 2013
//------------------------------------------------------------------------------

using System;
using Windows.ApplicationModel.Activation;
using Windows.Foundation;
using Windows.UI.Core;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace Splash_Screen
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class ExtendedSplash : Grid
    {
        public ExtendedSplash()
        {
            this.InitializeComponent();
        }

        /// <summary>
        /// Constructor with splash screen information
        /// </summary>
        public ExtendedSplash(SplashScreen splashscreen, bool loadState)
        {
            InitializeComponent();

            //
            // Aquí pondremos lo que haya que actualizar en los textos
            //
            if (DateTime.Now.Year > 2013)
            {
                txtCopyr.Text = "©Guillermo Som (elGuille), 2013-" + DateTime.Now.Year.ToString();
            }

            // Listen for window resize events to reposition the extended splash screen image accordingly.
            // This is important to ensure that the extended splash screen is formatted properly in response to
            // snapping, unsnapping, rotation, etc...
            Window.Current.SizeChanged += ExtendedSplash_OnResize;

            splash = splashscreen;

            if (splash != null)
            {
                // Register an event handler to be executed when the splash screen has been dismissed.
                splash.Dismissed += DismissedEventHandler;

                // Retrieve the window coordinates of the splash screen image.
                splashImageRect = splash.ImageLocation;
                PositionImage();
            }

            // Create a Frame to act as the navigation context
            rootFrame = new Frame();

            // Restore the saved session state if necessary
            //RestoreStateAsync(loadState);


        }
        
        //async void RestoreStateAsync(bool loadState)
        //{
        //    if (loadState)
        //        await SuspensionManager.RestoreAsync();

        //    // Normally you should start the time consuming task asynchronously here and  
        //    // dismiss the extended splash screen in the completed handler of that task 
        //    // This sample dismisses extended splash screen  in the handler for "Learn More" button for demonstration 
        //} 


        // Rect to store splash screen image coordinates.
        internal Rect splashImageRect;

        // Variable to track splash screen dismissal status.
        internal bool dismissed = false;

        // Variable to hold the splash screen object.
        private SplashScreen splash;

        internal Frame rootFrame;


        private DispatcherTimer showWindowTimer;
        private int showWindowTimerNum = 0;

        private void OnShowWindowTimer(object sender, object e)
        {
            showWindowTimerNum += 1;

            if (showWindowTimerNum == 1)
            {
                // Activate/show the window, now that the splash image has rendered
                Window.Current.Activate();
            }
            // aquí hacemos un pequeño descanso antes de mostrar
            // la página principal
            else if (showWindowTimerNum >= 50)
            {
                showWindowTimer.Stop();
                cargarMainPage();
            }
        }

        private void extendedSplashImage_ImageOpened(object sender, RoutedEventArgs e)
        {
            // ImageOpened means the file has been read, but the image hasn't been painted yet.
            // Start a short timer to give the image a chance to render, before showing the window
            // and starting the animation.
            showWindowTimer = new DispatcherTimer();
            showWindowTimer.Interval = TimeSpan.FromMilliseconds(50);
            showWindowTimer.Tick += OnShowWindowTimer;
            showWindowTimer.Start();
        }

        // Position the extended splash screen image in the same location as the system splash screen image.
        private void PositionImage()
        {
            extendedSplashImage.SetValue(Canvas.LeftProperty, splashImageRect.X);
            extendedSplashImage.SetValue(Canvas.TopProperty, splashImageRect.Y);

            extendedSplashImage.Height = splashImageRect.Height;
            extendedSplashImage.Width = splashImageRect.Width;
        }

        private void ExtendedSplash_OnResize(object sender, WindowSizeChangedEventArgs e)
        {
            // Safely update the extended splash screen image coordinates.
            // This function will be fired in response to snapping, unsnapping, rotation, etc...
            if (splash != null)
            {
                // Update the coordinates of the splash screen image.
                splashImageRect = splash.ImageLocation;
                PositionImage();
            }
        }

        private void cargarMainPage()
        {
            // Navigate to MainPage
            rootFrame.Navigate(typeof(MainPage));

            // Set extended splash info on Main Page
            //(rootFrame.Content as MainPage).SetExtendedSplashInfo(splashImageRect, dismissed);

            // Place the frame in the currrent window
            Window.Current.Content = rootFrame;
        }

        // Include code to be executed when the system has transitioned 
        // from the splash screen to the extended splash screen (application's first view).
        private void DismissedEventHandler(SplashScreen sender, object e)
        {
            dismissed = true;

            // Navigate away from the app's extended splash screen after completing setup operations here...
            // This sample navigates away from the extended splash screen when the "Learn More" button is clicked.
        }  
    }
}

Visual Basic:

Protected Overrides Sub OnLaunched(args As Windows.ApplicationModel.Activation.LaunchActivatedEventArgs)

    ' Para usar el SplashScreen personalizado                   (07/Ene/13)
    If args.PreviousExecutionState <> ApplicationExecutionState.Running Then
        Dim loadState As Boolean = (args.PreviousExecutionState = ApplicationExecutionState.Terminated)
        Dim extendedSplash As ExtendedSplash = New ExtendedSplash(args.SplashScreen, loadState)
        Window.Current.Content = extendedSplash

        ' ExtendedSplash will activate the window when its initial content has been painted.

        ' Salir
        Exit Sub
    End If


    Dim rootFrame As Frame = TryCast(Window.Current.Content, Frame)

    ' Do not repeat app initialization when the Window already has content,
    ' just ensure that the window is active

    If rootFrame Is Nothing Then
        ' Create a Frame to act as the navigation context and navigate to the first page
        rootFrame = New Frame()
        If args.PreviousExecutionState = ApplicationExecutionState.Terminated Then
            ' TODO: Load state from previously suspended application
        End If
        ' Place the frame in the current Window
        Window.Current.Content = rootFrame
    End If
    If rootFrame.Content Is Nothing Then
        ' When the navigation stack isn't restored navigate to the first page,
        ' configuring the new page by passing required information as a navigation
        ' parameter
        If Not rootFrame.Navigate(GetType(MainPage), args.Arguments) Then
            Throw New Exception("Failed to create initial page")
        End If
    End If

    ' Ensure the current window is active
    Window.Current.Activate()
End Sub

C#:

protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    // Para usar el SplashScreen personalizado              (07/Ene/13)
    if (args.PreviousExecutionState != ApplicationExecutionState.Running)
    {
        bool loadState = (args.PreviousExecutionState == ApplicationExecutionState.Terminated);
        ExtendedSplash extendedSplash = new ExtendedSplash(args.SplashScreen, loadState);
        Window.Current.Content = extendedSplash;

        // ExtendedSplash will activate the window when its initial content has been painted.

        // Salir
        return;
    }
    

    Frame rootFrame = Window.Current.Content as Frame;

    // Do not repeat app initialization when the Window already has content,
    // just ensure that the window is active
    if (rootFrame == null)
    {
        // Create a Frame to act as the navigation context and navigate to the first page
        rootFrame = new Frame();

        if (args.PreviousExecutionState == ApplicationExecutionState.Terminated)
        {
            //TODO: Load state from previously suspended application
        }

        // Place the frame in the current Window
        Window.Current.Content = rootFrame;
    }

    if (rootFrame.Content == null)
    {
        // When the navigation stack isn't restored navigate to the first page,
        // configuring the new page by passing required information as a navigation
        // parameter
        if (!rootFrame.Navigate(typeof(MainPage), args.Arguments))
        {
            throw new Exception("Failed to create initial page");
        }
    }
    // Ensure the current window is active
    Window.Current.Activate();
}