To check your upgrade eligibility, just dial *639# from your iPhone. AT&T will send you a text within a few minutes detailing how much you’ll pay to move up to the newest iPhone model — excluding the cost of the phone.
AT&T’s upgrade eligibility message
In my case, I’ll be paying US$18 in addition to signing a new two-year contract to move from my iPhone 3G to the new model.
This is a quick and easy way to check your upgrade eligibility, and AT&T doesn’t charge you for the text message.