While this "Jupiter" thing sounds like it should keep Silverlight, .Net and even C++ in the game for writing "immersive" apps, It's not clear to me whether the UI library it will provide is a superset or subset of SL or WPF, or something entirely new. Even though it uses XAML, that doesn't mean the classes have to be the same. It would also need to be usable from C++ and JS so it's possible it will be (next gen) COM based.