My second favorite Ajax library just added support for Silverlight. From this recent announcement it looks like Dojo is adding support for Silverlight in its 2D vector APIs. That is very cool to see!
Dojo 0.9 Final Version Released
DojoX high quality implementations of previously experimental features: gfx (portable 2D drawing), data wires, offline, storage, cometd, etc. dojox.gfx now includes Sliverlight support many more features and improvements than there’s room for here.