WPF 3.5 includes an exciting new feature: Viewport2DVisual3D. It makes building interactive 3D experiences very easy and is nicely integrated with the rest of the WPF Platform. I have used this feature to write a new version of the "Ink Postcard" sample that I wrote a few years back for the Windows SDK. A VS project that contains the new and improved version is attached to this post (you need .NET 3.5 installed to run it).
The sample allows you to draw ink directly onto a 3D model (a Postcard in this case) - even when it is animated! It demonstrates how well this new feature is integrated with things like Ink, Animation, InkCanvas and input from mouse and stylus (when running on a TabletPC).
More information on Viewport2DVisual3D and other great samples around this can be found on Lester's WPF Blog and on Greg Schlechter's Blog.