So MEF is not the same thing as PRISM. Prism is a UI composition framework, which can be used with WPF, Silverlight, WP7, Surface, and probably with C# based Metro Style Apps for Win8. On the other hand, MEF is a general component composition framework, and acts as a robust dependency injection solution. By composition I mean it can discover your components in runtime and wire up them together. MEF is particularly useful with its recomposition feature - basically the system can change its behavior in runtime. Imagine dropping a new dll into app's bin folder, that dll is immediately picked up and new functionality shows up!
When it comes to Silverlight, MEF works there like a charm! You can have prism modules loaded by MEF. Finally, MEF for SL comes with DeploymentCatalog - it allows you to download additional XAPs in runtime, and assuming you leverage the recomposition feature, you can add new functionality for the app as the user works with the app, thus you can conserve bandwidth on the app startup as you don't have to download everything at once.
So summing up, MEF works great with SL, and also with Prism. Highly recommended tandem!