Check out TruckWars, a Real-Time-Strategy (RTS) game for Silverlight 1.1 by Tim that builds on the Farseer 2d Physics engine. It's available on the gallery here.
Here's a screen shot:
You control the green trucks, and your goal is to push all the green buttons to clear each level. The red trucks are the bad guys.
The full source is also available on codeplex here, and Tim has some more detailed blog entries about aspects of this here.
There are some key takeaways here.