I'm thinking about purchasing an unlocked iPhone 4 off craigslist. The guy I'm talking to right now said AT&T unlocked the iPhone. I've heard that if you apply a software update the unlock will be removed. I'm on T-Mobile so I need the phone to be unlocked otherwise it's worthless. Is this true? What are the easiest ways to maintain an unlocked iPhone but have it running on the newest version of iOS?