Looks like there may be some XNA 2D games on they way to your browsers.  Here is a short blip:

http://arstechnica.com/microsoft/news/2009/03/xna-2d-games-come-to-the-browser-via-silverlight.ars

Here is a link to the SilverSprite library at CodePlex:

http://silversprite.codeplex.com

Will be interesting to see Silverlight move heavily into this market place.  While many are banking on business apps, the game industry should be pretty huge over the coming years.