ASK HN: How did they break in to my Gmail acct?

So I woke up today, checked my email and there's this whole list of undelivered mails, apparently someone or something had broken into my gmail acct and sent the following msg to everyone on my contact list:

Hey， how are you doing recently? I would like to introduce you a very good company and its website is www.ele-stores.com. It can offer you all kinds of electronic products that you may be in need,such as laptops ,gps ,TV LCD,cell phones,ps3,MP3/4,motorcycles and etc........ You can take some time to have a check ,there must be something interesting you 'd like to purchase . The contact email: elestores@188.com. MSN: ele-stores@hotmail.com

TEL: 0086+13717782599

Hope you can enjoy yourself in shopping from that company !

Regards

They also decided it was a good idea to put that msg into my signature as well as set my account to vacation mode with the same vacation msg.

Obviously this was a very brazen thing to do, and they clearly made no effort to cover it up, neither was my password changed. So I figure this is more likely an automated thing, I left my account logged in with a firefox 3 browser at work (university firewall) overnight, which is when this occured. So all signs point to some sort of an automated attack through the browser, does anybody know more about how this happened?