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 came across an open-source project called SlimDX the other day. It seems to wrap the DirectX API nicely and unlike XNA, MDX or the Code Pack projects from Microsoft, it covers 9, 10.1 and 11 (as well as Direct2D). They were also good enough to include samples of WPF interfacing, so if you want to use an immediate mode 3D API within a WPF application, I'd suggest checking that out because sitting on our hands waiting for Microsoft to get their stuff together with WPF doesn't seem very productive.
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.
People are going to want that - in fact it's something I've complained about a lot on this very forum - but the more I think about it, I don't see it happening at least with the current generation of phones. Silverlight is the most spectacularly inefficient way possible to display a user interface and it consumes so much of the phone's resources that I doubt it would be possible to do any XNA calls and still have things run at a reasonable speed.