Although Apple locks its phones to carriers as per exclusive and subsidized
agreements, those who braved jailbreaking have found ways to unlock phone to
work on other networks.
Unlocking tricks have persisted through all three previous generations of
iPhones, but now the unlock situation is a little more complicated as the iPhone
4 is a new piece of hardware that's quite different from the 3G and 3GS.
In fact, the current way that iPhone 3G and 3GS are unlocked in software
won't work at all for the iPhone 4 thanks to a new hardware and software system.