Thanks for bringing the topic up, I always wondered why some sites ask me for only my ZIP next to the CC number, but never really got into finding out why.
A few years ago, gas stations around here starting asking customers to enter their ZIP at the pump. Not long after they started doing that, many added a clarification: billing address ZIP, not residence ZIP. Apparently, some customers thought they were asking for market-research reasons, as opposed to credit card security.
