Today Glenn Block announced that the first source bits of the Managed Extensibility Framework are out on CodePlex now.

This looks like a really interesting project. I think it could be very useful for some projects I'm currently working on. Tonight I'm going to explore the source a little bit. I'm especially interested in how it could be leveraged with Smart Client applications that are based on Prism.

From the project site:

The Managed Extensibility Framework (MEF) is a new library in .NET that enables greater reuse of applications and components. Using MEF, .NET applications can make the shift from being statically compiled to dynamically composed. If you are building extensible applications, extensible frameworks and application extensions, then MEF is for you.