To my understanding, you can buy an iPhone for the full retail price without getting an AT&T plan. Can this be done online, or does it have to be done at an Apple store? Does unlocking an iPhone void it's warranty? Do I have to buy a phone with T-Mobile, or would I be able to like take the iPhone 4 into their store and get a contract/sim? If no to the second part of that, would they terminate my service if they found out it was being used with an iPhone?