After upgrading from VS Team System 2005 to VS 2008 with Team Explorer I had a TestContext error when trying to run a unit test. I found the fix to the problem here: http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2459840&SiteID=1

 

You have to change the reference to Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll from v8 to v9.