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.

Read it here.

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!