I am pretty excited to see MEF preview 3 ship on codeplex today! Please check out Glenn's post for all the details... This is a release where we really focused on getting in your feedback... We are 100% MSPL, we are shipping our own unit tests, we have made several design changes based on feedback around defaults, ExportProvider, etc. One thing I love about taking on a big customer such as Visual Studio, is you get good, real world feedback up front. In this CTP, we really focused on performance and performance best practices. Whether you use MEF directly or just use Visual Studio, you will appreciate this work!
So grab the bits and let us know what you think!