It's been fixed, but here's what it was:
http://www.elhacker.net/gmailbug/english_version.htm
That's scary.
Just goes to show, maybe you should thank people properly if you don't want them to get mad at you and do something you may just not like (reference: last paragraph).
Interesting, I wonder how they figured out something like this. It is by no means a simple little exploit that one might just happen on.
Hoho, that's whicked. Props to Google for being leet and fixing this.