Check the add-ins you installed before Visual Studio 2010 if you enabled VS 2010 integration in them. Particularly TestDriven.NET is the problem. Go to “Add Remove Programs” / “Programs and Features”, find Test Driven.NET, click “Change”, and then modify it to remove VS2010 integration from it.
Also, if you are using AnkhSVN, make sure to download a new latest daily update build and use that, otherwise, you’ll get an exception when starting VS 2010.
I’ll be writing more about VSTS 2010 and .NET 4.0 in the next coming months (if God will), since now I can run it :D (before, I did not have the fix that extends the Virtual Machine expiration after January 1st, 2009).
BTW, for R# deep lovers (like me), Resharper for VS 2010 is expected next month.
Have fun, please!