Comment AT&T is doing it now (Score 2, Informative) 215
I just got a refund from AT&T because of an issue like this with my iPhone 4.
I turned all data access off (e.g. if I didn't have WIFI access I would get a dialog about cell data being off) and yet I was getting hit for 0.8MB/day while it was off. According to the AT&T person, it is because the iPhone sends out data to see if the data service is available! There are even discussions about it on Apple's site.