It's been a while since I've talked about the Managed Extensibility Framework, but there were two recent announcements: a new preview release and news that MEF is coming as a part of Silverlight 4.
The Managed Extensibility Framework makes it easier to create and reuse extensibility 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. We previously had an implementation of workflow services whose extensibility model used MEF, but the schedules ended up making it too hard to take the dependency because we're shipping as part of .Net 4.
MEF is going to be available as part of Silverlight 4, although you can get a preview release now for both .Net 3.5 and Silverlight 3.
You can also get an overview of MEF in Silverlight from Glenn Block's PDC talk.
Building Extensible Rich Internet Applications with the Managed Extensibility Framework by Glenn Block
Learn how the Microsoft Silverlight 4 introduces the Managed Extensibility Framework (MEF) for building customizable applications that can easily be extended by third parties. Whether you are building an extensible data grid, a custom rules engine, a pluggable editor, or a composite application such as a pluggable CRM system, you want to learn about MEF. Hear how to use MEF to decouple your applications into more maintainable and testable pieces, and partition your application into dynamically deployable chunks that download on-demand.