MacOSX can sometimes corrupt a drive (like most OSen).
I've seen it happen a few times, in a couple of different ways. One way is if you interrupt some file transfers. Like by pressing 'stop' on a big transfer - this can trash your partition table. You'll likely see vfs errors in your log at this point. Anyway... linux, ubuntu and 'testdisk' came to the rescue. testdisk found the partition for me, and wrote it back... luckily it worked and my friend and I did a happy dance.
The HFS+ partition was saved.