A number of IoC containers have Silverlight versions - Ninject and Unity especially seem to have healthy Silverlight offerings. Silverlight 2.0 is a variant of the .NET runtime, so this isn't too surprising.
Thanks to Tyson Stolarski and Rinat Abdullin, Autofac recently joined their ranks with a Silverlight-compatible build. (This one is hot off the press, so test it thoroughly before you set out to build anything mission-critical :))
MEF hasn't released any concrete plans for Silverlight, but experimenters can probably coerce the Codeplex source into a compatible build by hacking the .csproj file.
If you're reading this blog and use IoC on Silverlight in one form or another:
I'd love to hear your thoughts!