@Ian2:As far as I can see, Jupiter is not Silverlight nor WPF, its another kind of framework that kinda similiar or compatible to Silverlight/WPF code.
so Sinofsky is right, that 'Silverlight', as we know it, is still a Browser/Desktop thing.
what he didn't say is, the 'Jupiter' framework will enable you to leverage your Silverlight/.NET skills to write Immersive (or desktop) applications in XAML/C#/VB/C++, its very very similiar, but not exatcly the same thing.
the situation is similiar to the relationship of Silverlight and WPF and Silverlight for Phone, you need to create different project in VS, then you can write very similiar UI/code, even sharing most of your portable code in a common library, but the underlying technology are still very different.
that said, I think you can easily port a 'real WPF' app to the immersive ui, just host your WPF content inside an immersive HWND, maybe you can play some trick with 'real' silverlight too, with COM interop or P/Invoke. Hmm.. maybe Microsoft can have these little tricks offically builtin but that wont make much sense, because in the end of day, you still need to call some new apis to provide a real native experience, so what you get is just a hybrid monster, why not just use Jupiter ?
and, the Silverlight is portable, and the immsersive experience is very unique and not portable, you cant use them in the browser or on a Mac, it doesnt make sense to add those into a portable technology. the only exception may be the phone. but Apollo is the son of Jupiter IIRC.