I remember seeing a technology demo awhile back where someone on the WPF team was demonstrating an application that used both xna and wpf. that is to say the background of the window was an xna surface but wpf controls were being composited on top. I also recall Shawn Hargreaves hinting that they were looking into ways to be able to use silverlight and XNA in the same application on WP7.
It would be super sweet if we got a XNASurfaceControl for silverlight. We could write games and use the same code to compile for desktop, web, xbox 360 and wp7. now that'd be sweet. Would see games in silverlight erupt online too i bet ;].