IS THAT REALLY POSSIBLE?!?! they don't check to make sure it's the same computer? (mac address and whatnot?) I thought that may have been cheating the system and the last thing i wanna do is get kicked off by VAC.
Well, i can't really see how they could make sure it was the same computer. They can't use the IP because anyone with a dynamic IP or anyone who switched ISP's would be screwed.
Though I am not an expert, I think if someone gets a hold of your steam account they are able to use it however they like, so long as two people don't try to sign on at the same time, or use a steam hack.
*EDIT* Plus what KFS said 
[addsig]