I updated Bill Reiss demo on SpaceRocks to include firing at the asteroids. If you have not checked out his demos's yet, be sure to visit SilverlightRocks. He got it all started, and I just added one extra step ( Keep em' coming Bill! ). I added the XAML for a simple laser bolt, and am treating it just as the other sprites (ship/asteroids) with additional checks for collision. The keys A,D,and W control the ship. Use the space bar to fire.
Note: This demo requires Silverlight 1.1.