Chris Bowen, a Developer Evangelist for Microsoft has compiled a list of Silverlight 1.0 & 1.1 games that are currently available, each (but one) with source:

• "Dr. Popper" [source] - Burst the matching bubbles, the more, the better.
• "Digger" [source] - Sort of Dig-Dug meets Montezuma's Revenge.
• "Zero Gravity" [no source available yet] - An addictive and well-made game!  A nice example of the possibilities of the platform.
• "Chess" [source] - Your basic chess, but shows how Silverlight 1.1 with .NET fares against a JavaScript-based AI.
• "Sprawl" [source] - A good demonstration of potential with Silverlight 1.0.
• "Destroy All Invaders" [source] - A cool mashup with Virtual Earth where you fly over the oblique angle view of a location of your choice while shooting UFOs.
• "Silverlight Rocks!" [source] - A shoot the asteroids game.

If you're interested in learning more about game development with Silverlight, Bill Reiss has been working on a great series of tutorial posts on his Silverlight Games 101 blog.  Start reading from the bottom first, where he start with a basic game canvas, then adds sprites, a game loop, keyboard input, etc.