The 3.0 and 3.5 .NET libraries were just add-ons to 2.0. So it ws possible to test work done in VS 2008 using VS 2005 if you didn't use any 3.0 or 3.5 features.
My understanding is that the 4.0 is entirely new libraries, so I doubt you will be able to load or use them in VS 2008/2005.