To what extent, if any, is MEF a replacement for PRISM?
Edit: Do not read this answer. It is embarrassingly wrong. I am fail. Read Glenn Block's below.
It's not obvious, but this is the same question: Managed Extensibility Framework (MEF) vs. Composite UI Application Block (CAB)
Consensus in the duplicate post is that MEF and Prism provide the same basic set of functionality in different ways, except that Prism provides the Event Aggregator, which is a pub-sub means of communication between application components. You can use this with MEF, however. It's pretty much up to preference, really.