Could this be the first ever 3D Silverlight game?
My colleague Ito ported the rail shooter that he originally wrote for AppWeek 2010 to use the new 3D features in Silverlight 5. First get the SL5 RC plugin (if you don't have it already). Then play the game here.
Once the intro animation finishes playing, mouse over enemy ships to target them, and left click to fire.