Vista - WPF interop with DirectX and OpenGL?

I understand the WPF has interop capabilities that allows a win32 HWND
to be embedded in a WPF application and vice-versa but because WPF
implements its compositing using DirectX9 (and a win32 HWND could use
e.g. GDI) these windows cannot overlap without suffering nasty
rendering artefacts.

What is the extent of this? E.g. is is possible to create a WPF frame
window and embed a DirectX rendered win32 HWND (assuming there are no
WPF context menu type pop-ups that would cause WPF elements to overlap
the hwnd)?

Similarly could such a window use OpenGL instead of DirectX?
Even under Windows XP?

I'd appreciate any insight into the practical limitations of these
issues.
Thanks!
-
Robin Charlton