Thought only the iPhone 5 with Verizon was having this problem, but after going through 4gb data in one week, I called AT&T. Tech support said it is a "known issue" and they are waiting for Apple to release a software update. In the meantime they suggested manually turning off cellular data when on a wifi network. They then gave me, without much prompting, a $50 bill credit. Hope Apple will fix this soon.