Right now, the iPhone can only be used with the cards they come with. Another AT&T card won't even work.
Most other phones have a limited exclusivity on them - usually 3 months or so before you start seeing variants show up with the other carriers. Kind of how the BlackBerry Pearl was exlusive to T-Mobile for a little while and the Curve is now exclusive to AT&T. Rumors are that the iPhone exclusivity is 3 YEARS and that Apple gets revenue sharing from the monthly service (varies based on whether the subscriber is a new AT&T customer or an existing one that upgraded).
There are financial incentives for both Apple and AT&T to keep it locked down.