iPhone: Fade transition between two RootViewControllers
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`Obj-C` or `MonoTouch C#` answers are fine.

The initial UIWindow's RootViewController is a simple login screen.

    window.RootViewController = loginScreen;

After login, I set the Root to the main app

    window.RootViewController = theAppScreen;

How do I Fade-transition between the two RootViewControllers in this instance?

