i am having the same problem.
I have an application named MRPTools. It was originally compiled in AutoCAD 2006. I copied the project and removed the 2006 references, then added 2007 references, and re-built it. It worked just fine. Later, instead of managing two sets of code, i decided to Just use the 2006 project, and when i wanted to compile in 2007, i would just remove the 06 references, and then add the 07, compile the code. then remove the 07 references and add the 06 again. No big deal.
But now, i can't get add the Interop references at all !!!!!
I have tried everything, from building a completely new project, selecting the reference files themselves, physically copying the files into the project directory..... Nothing works!
This hasn't become a big deal yet, but it will, as i am constantly updating my programs for the company. We have come to rely on them very heavily, and now the people using 07 will not be able to get updates.