A new version of the Wpf3D open-source class library, now included in a library called Perspective, is available on Codeplex.
Perspective is an experimental .NET 3.5 class library for building WPF 3D models based on basic geometrical objects (polygons, cube, sphere, ring, etc.). 3D classes inherit from UIElement3D so it is now easy to build interactive scenes.
For a general introduction, you can read Perspective : easy 3D programming with WPF.
See you this week in Barcelona [:D] to speak of it, on the Ask the experts / .NET Framework booth at TechEd Developer Europe.