By PETER SVENSSON, AP Technology Writer – Wed Jan 26

NEW YORK – Verizon Wireless will start offering the iPhone on Feb. 10 with a draw that AT&T Inc. no longer offers to new subscribers: a plan with unlimited data usage. But The Associated Press has learned that some AT&T iPhone users on limited plans won't need to move to Verizon for all-you-can-eat data.
In an unadvertised loophole, AT&T has allowed subscribers who have had an unlimited data plan in the past to switch back. That includes anyone who had an iPhone before June, when the limited plans took effect.

Jose Argumedo, of Brentwood, N.Y., says he and a friend were switched to an unlimited plan recently after they called AT&T's customer service. Both have iPhone 4s, and previously had earlier iPhone models.

AT&T spokesman Mark Siegel wouldn't confirm the option to return to an unlimited plan. "We handle customers and their situations individually, and we're not going to discuss specifics," he said.

The company has allowed iPhone and smart-phone users with unlimited-data plans to keep them when upgrading directly to a new phone. It's the option to return to an unlimited plan after going limited that's been secret.

Spotted here