How to stop Visual Studio from “always” checking out solution files

tfsvisual-studio-2008

For apparently no reason, every time I open my solution, Visual Studio checks the sln file out.

If I compare it to the previous version there are no changes. But this is really frustrating since everybody has the solution checked out.

I'm using VS 2008 and TFS 2008, both SP2.

Any idea on how I can stop this thing to happen? Or is a feature/bug of the TFS source control provider for VS?

This happens when the following is in the .sln file:

GlobalSection(ExtensibilityGlobals) = postSolution
    MyGlobalProperty = AnyValue
EndGlobalSection

I found that Enterprise Library added one of these. I removed it, checked in the solution, closed then re-opened it, and no more automatic check out.