We have just released an update to MEF. You can get it at http://www.codeplex.com/MEF
The changes are quite significant:
1. The preview ships with sources under a very permissive license (Ms-LPL).
2. We now support constructor injection. Feature that the community asked for.
3. We completely redesigned MEF’s extensibility points. The extensibility points are designed to support writing custom providers of composition data. For example, out of the box MEF requires composable parts to be attributed with attributes that provide metadata describing the composition. We got lots of feedback that this is not acceptable in many scenarios. The new extensibility points make it easier to extend MEF to externalize the metadata (to an XML file for example). Note, that the changes are just the first step toward the goal of making the extensibility easy and powerful. We will most probably keep making improvements in this space in the future, so feedback on the new extensibility points would be more then welcome.
4. We significantly cleaned up the container APIs. But as above, there is more clean up to come in the future.