I'm looking for a more detailed post but haven't found anything yet apart from the simple example on the BCL blog. I have asked a question about how to us CompositionScopeDefinition on stackoverflow.
http://stackoverflow.com/questions/16943121/defining-scope-in-mef-with-compositionscopedefinition
http://stackoverflow.com/questions/16943121/defining-scope-in-mef-with-compositionscopedefinition