I don't have a solution (still looking), but I do have some advice. Before you start anything as drastic as database restores, try simply restarting the server. When I've encountered this error, a restart has been sufficient--it clears whatever file handle was preventing SQL Server's access to the file.

I have not yet been able to figure out what was causing the error in the first place. Process Explorer seemed to be saying that SQL Server had the only handle to the file.