Blog post explaining how you can enable a DSL to MEF extensions
Jean-Marc published a nice post on how to enable a DSL to MEF extensios. There's a followup post for those who migrated from VS2005 and missing the magical InternalsVisibleTo attribute in their AssemblyInfo.cs.
For more info on DSL tools see this code gallery page: http://code.msdn.microsoft.com/vsvmsdk
Eyal