I don't see any technical reason for not being able to do GUI using "retained" (xaml) style while doing the engine in more "immediate" (xna) style. It should all be built into Silverlight/WPF and require no pinvokes from the API consumer. Of course it would be nice to have some way to add those guis to XNA apps as well and all this stuff should load real quick. Smiley I think it's doable.