Solution: delete OPT and NCB files with the same name of your workspace (you find them in the same root folder of the workspace - back them up for safety if you feel so), they'll be re-generated as soon as you open the workspace.
This is one of those nasty ones you could spend a couple of hours on without having a clue. Those two kids just get corrupted sometimes/somehow (especially if you move the project, do some changes on some other machine/environment and then go back to yours). I tried to google it but no luck; just found the microsoft error page ( http://support.microsoft.com/kb/166389 ) which won't help you to solve it. I discovered the solution by empirical method (read RANDOM trials).
Talk Ya'll soon.