AT&T have just come out as the good guys today, where they've issued a press release announcing that you can keep your grandfathered unlimited data plan, even if you move over to the new iPhone, or upgrade using its subsidized pricing.
AT&T's Ralph de la Vega talked about the iPhone and its upgrade process:
We offer customers the flexibility to keep the iPhone data plans they already have or choose any of our individual or new Mobile Share plans. We're proud that more customers choose AT&T for iPhone than any other U.S. carrier and look forward to making iPhone 5 the newest addition to our lineup.
That's pretty great of AT&T to do this, considering the new iPhone sports LTE connectivity for super-fast Internet access. Now, remember that unlimited with AT&T's unlimited plan only goes up to a point, which is 5GB. Once you reach 5GB you'll be throttled until your next billing cycle.