I got bit with an issue that I’d love to spare you.
For a short time MEF Preview 8 was posted with a fairly serious bug for Silverlight programmers using PartInitializer.
The way PartInitializer should work, and works now, is that all exports within the current XAP should be discovered by default.
When MEF Preview 8 was first posted, it included a bug which resulted in the XAP not being searched for exports – no exports except those explicitly placed into the container were discovered. The incorrect version was up for a handful of days. Of course luck would have it that I downloaded during that time, and a spasm of insecurity led me to believe I’d forgotten how to use MEF.
Here’s Glenn Block’s discussion.