In July last year I wrote a blog post, and associated connect item, where I discussed the possibility of executing Visual Studio Team System unit tests in parallel upon multi-core architectures. 

It seems that there were others on the Visual Studio Team Test team that also thought this was a good idea and Bruce Taimana, a program manager for Visual Studio Team Test, has written an excellent blog post where he describes how this can be done within Visual Studio 2010.

In the original blog post that I wrote, I stated that "Obviously thread safety would be the responsibility of the test authors and the parallel execution of the unit tests could be an optional capability with the default being the sequential execution."

As Bruce Taimana indicates in his blog post, thread safety is the responsibility of the unit test authors and he provides a link to an article within the MSDN magazine that addresses thread safety.