We’ve been enjoying injecting MEF extension points in our tools to make it easy for customers to write extensions using very simple code. Jean-Marc has just posted a sample demonstrating how MEF-enable your own DSL so you too can add MEF extension points for customers. See http://blogs.msdn.com/jmprieur/archive/2010/05/17/enabling-your-dsl-for-mef-extensions.aspx for details.