Maybe it was in the 50 some page User Agreement no one really reads and just signs off to get their phone? Only in America... such simpleminded-ness is cute
Anyways, as many have said eyeseeyou, you can't use the phone without the high probability of ATT noticing and tacking you with a $15 a month data plan.
To my knowledge, the only two things you can do is either one, use it as a full fledged iPhone with data costs and the whole package, or two, essentially turn it into an iPod Touch by hacktivating and removing the sim card [which obviously means absolutely zero possibility to make calls].
It really depends on the account. It can be done, but it rarely works.
It's not going to work on a new account. Only old accounts that were upgraded to smartphones without a data plan (when AT&T didn't require them) and haven't been upgraded since.