Vince (who I knew in high school as Steve) wrote in my comments:
How can I get to the underlying DX9-esque interface I know you guys gotta be using this stuff. I don't want to deal with EllipseGeometry objects and the like, gimme DrawPrimitive, push buffers, and pain-in-the ass ITexture objects.
Don't worry dude! The details are still being worked out, but we know that we have to make it possible for people to interop with old code, and with DX. At the very least, you will be able to host another hwnd window that is written using GDI/User or DX. The details on when and how DX will get accelerated, and how it will play with the rest of the system are still TBD. At the very least, when you go full screen with DX you will own the device.
For simple 3D stuff (load up a model and spin it around like a 3D icon) we will have our own 3D API that doesn't pretent to offer all of the flexibility of D3D.
What specific scenarios are you interested in? My thinking is that you might want to write a tool and have the UI done with Avalon and the "canvas" of the tool done via D3D. In this scenario, you are in a windowed situation and Avalon is on top. The 3D stuff is running in a window in an Avalon app. Another scenario that might be interesting is that you have a D3D app/game and you are using Avalon to drive 2D type UI inside of the game environment.
I can't promise that all of the scenarios will be doable, but let me know what is most important to you.