Why does Visual Studio create a new .vsmdi file?

Question

If I open a solution in Visual Studio 2008 and run a unit test then VS creates a new .vsmdi file in the Solution Items folder and gives it the next number available e.g. My Solu

Answer 1

I work around this by always checking out the .vsmdi.
It seems that this only happens when the .vsmdi file is read-only, e.g. not checked-out in a version control system that uses that kind of lock-local-files behavior (Perforce etc).