A few days ago we released MEF Preview 5 on codeplex with all the source code, docs, test and samples you need to get started.  While MEF will be a core part of .NET Framework 4, we are making these preview releases available so folks can use MEF today on top of .NET Framework 3.5. 

I have been very happy with the feedback we have gotten so far on MEF and I hope you see that we are reacting to it. 

Glenn has the best list of changes and enhancements, but Wes has some good tips on moving to Preview 5 as well!