Check out Bubble Factory by Vincent Vergonjeanne who in four days went from no knowledge in Silverlight 1.1. to writing this pretty cool little game: http://cosmik.members.winisp.net/BubbleFactory
It's a fast frenetic puzzle game where you have to beat the computer (which is fiendishly difficult to do!).
Another terrific game is this one, Destroy all Invaders, which is a terrific use of Silverlight as a mashup with Windows Live services. Here, the author, Andy Beaulieu has built a Silverlight game that allows you to save the world from Alien invaders. As the screenshot shows, you can enter any address, get the arial photo of that address and shoot down the bad guys. Here is a screenshot of me flying a helicopter over my neighborhood. Check out the semi-transparent map at the bottom right, and the helicopter shadow (over my neighbors garden) below and to the left of the helicopter. I hope for V2 that he will allow us to shoot up our neighbors too 🙂