David Hill has a recent blog post that discusses how Prism 4.0 (Composite Application Guidance for WPF and Silverlight) and MEF will work together. You might be thinking what happened to Prism 3.0. We decided to call the next version of Prism to be 4.0 as it will support .NET 4.0 and Silverlight 4.
In the blog post, David also mentioned some of the frequently requested items that we heard for Prism vNext. We will start the project early 2010.