Here's the post that was supposed to appear last Friday.
The Managed Extensibility Framework makes it easier to create and reuse extensible applications by providing a standardized model for discovering and composing application plugins. This allows you to get an off the shelf component model without having to build the infrastructure yourself as part of your application.
This one download includes binaries, source code, and product samples. Besides a new sample the primary changes in this preview are for Silverlight and fixing bugs.
MEF is going to be available as part of Silverlight 4, although you can get this preview release now for both .Net 3.5 and Silverlight 3. For Silverlight 4 and .Net 4, use the version of MEF that comes with those beta releases instead.
And for more details about this preview release, see this article on MEF Preview 9 by Mike Taulty.