Visual Studio Workspace Issue
Apparently the same workspace name for two different TFS servers is an issue. I downloaded a new project from the codeplex site, and gave it the same workspace name as a different TFS project. When I opened the project that has had no problems for years, I started getting "all files must be in the specified workspace. Workspace <name> contains <folder>." which was real strange. Of course it was there, I just created it at the time; I got this when I added a new file to the project.
Quickly changing the workspace name to be unique solved the problem.