I have a project with several unit tests that require windows/dialogs to be displayed in order to exercise my WPF-based classes. Until VS 2008 RTM (i.e., with Beta 1 and Beta 2), these tests all passed successfully. In VS 2008 RTM, the test host (whether VsTestHost or MsTest) both crash on shutdown, with the exception message: "COM object that has been separated from its underlying RCW cannot be used." This is a blocking issue in that the test host also returns a non-zero exit code, causing failed automated builds (even though all the tests in fact pass). The only solution we've found is to remove the failing tests.
Note: A similar problem was reported earlier with VS 2005, under ID 97820.