VS 2005 RC build # 50727.26 has an issue related to running tests that causes a nasty "object is an invalid state" error message. If you are hitting this problem, here's some more background information in Q&A format:

Question:  Trying to run any test, from Test manager or Test View, we are getting a message saying something along the lines of "Object is in an invalid state" and the tests won't run, is there a workaround or fix for this problem?

Answer:  Fixed in later builds, workaround for now is to delete the .suo file and then reopen the solution.  The .suo file is designed to contain user-specific settings (if you delete it you might lose some user specific information but nothing you can't re-add on a per-user basis next time you open the solution).  It should be on disk in the same directory as the .sln file.

Here's the forum post on the topic: http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=96352