Charles Petzold has been generating many interesting blog posts dealing with WPF/XAML.
As part of his book writing effort (Applications = Markup +Code), he even created a XAMLPad like application called XAML Cruncher. It added a number of interesting features over XAMLPad (open, save as, etc...).
He just made his first updates to the application, and when I ran it I was prompted to see if I wanted to install the update. Nice way to keep software up to date.
If you want to try XAML Cruncher, install it from this page. (It requires the FebCTP of WinFX).
Charles - I'm looking forward to your book.