Tip of Monday, Access the current dispatcher with WinRT : http://www.sandor.fr/2012/11/26/lastuce-du-lundi-acceder-au-dispatcher-dans-une-application-winrt/

DispatcherW8-Clean.cs

await Windows.ApplicationModel.Core.CoreApplication.MainView.CoreWindow.Dispatcher.RunAsync(new CoreDispatcherPriority(), action);

DispatcherW8-Fail.cs

Window.Current.Dispatcher.RunAsync(priority, action)

DispatcherW8-Tired.cs

static CoreDispatcher dispatcher;

public static CoreDispatcher Dispatcher
{
    get { return App.dispatcher; }
}

protected override void OnLaunched(LaunchActivatedEventArgs args)
{
    Frame rootFrame = Window.Current.Content as Frame;

    // [.... initialization code]

    dispatcher = Window.Current.Dispatcher;

    // Ensure the current window is active
    Window.Current.Activate();
}

DispatcherWP7.cs

System.Windows.Deployment.Current.Dispatcher.BeginInvoke(action);