Apple May Be Breaking the Law With Policy On iPhone Unlocks
HOWEVER other functions of the iPhone (such as Visual Voice Mail) only work correctly on AT&T. The whole "registration" process only works on AT&T... you're buying a packaged product with functions other than a phone.
Sure the product works adequately on other carriers, but it does not work as Apple intended. Unless other carriers are willing to invest in the infrastructure required for the iPhone to function correctly Apple are in the clear, obviously no other carriers were willing to invest (at least as much as AT&T were willing to invest) - otherwise they would have negotiated a deal with Apple prior to the launch.