Brad Abrams wrote about the just released Framework Design Studio. Also there is not that much functionality included right now it already helped me to identify API changes between two milestones in an internal WPF framework we currently work on.
http://blogs.msdn.com/brada/archive/2008/04/04/framework-design-studio-published.aspx
In addition to the program itself you can download a 9 pages long excerpt from the book titled Framework Design Guidelines.
-t