Yep, that makes total sense, and that is what we did initially.
But when both versions are installed (specifically the x86 version), we get the error I talk about in the top post.
I tried it on a fresh XP x64 install, only visual studio 2008 team suite was installed. I can run tests just fine (just simple tests, create a class, create a test project for that class). As soon as I installed TypeMock (x86 and x64), I can no longer run tests with the error above (not even using TypeMock code in the tests). If I uninstall the x86 version I can run tests again.
I did try the same test with 4.1.0, and everything works fine.