Rob from the Public Sector Virtual Earth team just sent me an email pointing to a game called "Destroy All Invaders" where you fly around in a helicopter shooting down flying saucers that have invaded the earth.  The game was created by Andy Beaulieu.  He uses Virtual Earth as the backdrop for the game and Silverlight for the rest.  Pretty cool!  You can play the game at  Andy made the source code available via his blog here.