I am using Visual Studio 2010, try to create a MEF application. Does this require .NET 4.0 or can I target .NET 2.0?
Yes you do:
Application requirements change frequently and software is constantly evolving. As a result, such applications often become monolithic making it difficult to add new functionality. The Managed Extensibility Framework (MEF) is a new library in .NET Framework 4.0 that addresses this problem by simplifying the design of extensible applications and components.
From: http://mef.codeplex.com/
Edit: The question about targeting .net 2.0 is answered here:
http://mef.codeplex.com/Thread/View.aspx?ThreadId=54008
MEF is not supported on .NET 2.0 as it depends on LINQ and Expression trees. Currently we do not have a .NET 2.0 version of MEF planned.