Greg Schecter from the WPF/DWM team has made an interesting post about overlaying 3D content over 2D elements. This is to transition between different states of the UI by using 3D effects.
I personally feel that UI technologies should be 3D by default. 2D would then be just a special case with the appropriate positioning of the Camera in the Viewport. I think its our legacy with 2D interfaces which makes us think that there should be a separation between 2D and 3D. Some of the best UI experiences involve some 3D elements. Having a programming model where developers have to constantly keep switching between 2D and 3D makes it cumbersome (unless someone writes an abstraction layer). With a ground-up 3D UI technology it would be much more simpler and lead to a far more consistent and productive developer experience. May WPF v3.0 be all 3D!