Total myth that has no basis. AT&T has the right to cancel coverage that they consider excessive, but the current iPhone data plan (for AT&T) is unlimited (they'd have a hard time in court if they did try to terminate, IMO). There are so-called "soft data caps" that people talk about on here, but again they are trying to use stuff that is for other plans (air cards) and justify that AT&T would use those limits for the iPhone (again no basis in fact and stated nowhere on any AT&T site or from any memos from AT&T).