How to reference .NET 4.0 assembly within .NET 3.5 projects

Question

I would like to do some Office Interop stuff using .NET 4.0, but developing the rest of my project using VS2008 and .NET3.5.

How can I now use the functionality from

Answer 1

CLR 4 allows InProc Side by side, but you'd still be looking at some interop solution between the loaded 2.0 and 4.0 code.