I updated an old post: iPhoto library sharing - the official Apple method. There's still no good way to share iPhoto Libraries between multiple machine users. The best official workaround is using a disk image, which causes backup problems (grrr, Retrospect!).
The problem is not iPhoto's. I believe it's the result of an ancient design decision in BSD Unix file sharing. It impacts all resource sharing between users, including iTunes, iMovie, etc.
I wonder if it's addressed in Snow Leopard.
I think XP/Vista may have a real advantage here ...