Working on Bing Map Apps has gotten me a little deeper into Silverlight than I had been before. It wasn't long till I found the Farseer Physics Engine and Andy Beaulieu's Physics Helper for Silverlight, WPF, Blend, and Farseer. Check out http://farseerphysics.codeplex.com and http://physicshelper.codeplex.com .
It wasn't long before I took a crack at creating a behavior to add to the physics library. This behavior allows the user to move physics objects with the mouse. Just adding the default mouse move doesn't play well at all with the physics engine. You can find the code over in this thread.
I really love the event-driven, object-oriented way this whole system works. I was a little put off by XAML in the past because it was a pain to create by hand. Expressions Blend really makes this part easy. It's exactly how a tool for WYSIWYG XAML editing should work. If you are interested in building your own behaviors for Expressions Blend, the SDK documentation is right on the help menu, but the examples are really sparse. Being able to see how Andy did his code really made it make sense for me.