This issue has popped up for some after having upgraded from Team System Beta 2 to RC. You have remembered to uninstall everything (as Jakob points out here: http://geekswithblogs.net/jakob/archive/2010/02/09/reember-to-uninstall-all-beta-2-stuff-before-upgrading-to.aspx but you still get the following error box:
saying Could not load type 'Microsoft.TeamFoundation.Client.TeamFoundationServerBase' from assembly 'Microsoft.TeamFoundation.Client, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
At the same time, you see Power Tools functions are still there in Visual Studio, but no Power Tool appears in the Add/Remove programs. This is some freak happening when uninstalling the beta Power Tools.
The solution, as first suggested and tried by Mathias Olausson, is to simply install the beta 2 Power Tools again, and uninstall them again. That seems to solve the problem.