Buying an iPhone in the US? Pay full price
medium.com
medium.com
Walk into a USA Apple store, ask for a "device only" phone. They'll give you one of the AT&T stock, but when you get back to the office or home and connect to iTunes, you'll get the message:
“Congratulations, your iPhone has been unlocked.”
Tada, use any plan you like, anywhere you like. This worked on the iPhone 4S before Apple even announced no-contract phones.