p&p is looking to leverage MEF fully as part of Prism 4. MEF and Prism definitely can be used together.
The key areas where MEF plays in are:
1. Modularity - MEF provides a great mechansim for partitioning your applications into modules. A part basically is a module in MEF terms in the sense that it is dynamically discovered similar to IModule in Prism. This offers more flexibility from a modular
perspective than today is offered in Prism.
2. Composite UI - MEF has an interesting angle for doing some aspects of UI Composition. You can for example use metadata in MEF for designating which parts of the UI a particular component belongs to, and then have a manager which pulls in all the pieces
and composes them.
You can read more about the Prism / MEF direction on this post:
As far as meshing existing Prism bits with MEF today, it can fairly easily be done. Two approaches that seem to be quite common are pulling the RegionManger, EventAggregator and commanding into MEF. You can easily derive from the RegionManager or EventAggegator
and add exports in order to put them into MEF's container. This will allow you to leverage Prism's composite UI mechanisms as well as cross part communication services.
You can also roll your own module loading mechanism with MEF. For example here is a simple snippet that shows how one could load modules:
Keep watch on the p&p site / David Hill's blog as you are sure to see MEF/Prism stuff coming soon.