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.
According to ModMy, David Wang, one of the members of the iPhone Dev-Team that develops jailbreaking software, wrote:
We don't doubt that hackers will find a way. For now, at least those in Canada, France and the UK can buy factory unlocked iPhone 4 from the Apple Store – whenever they launch.