AT&T sent out a press release that says that you will be able to keep your old unlimited data plan and grandfather it into the iPhone 5 with LTE capabilities even if you upgrade with the subsidized pricing. AT&T Mobility president and CEO Ralph de la Vega said the following:
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.
The download speeds on AT&T generally starts getting throttled when the unlimited data hits around 5GB though. Verizon lets you keep old unlimited data plans, but is refusing to subsidize their phones. You will have to pay the full rate for the iPhone 5 if you want to keep your old plan.