It's not clear if he's referring to a prepaid plan here. Prepaid definitely is the safest bet - essentially no surprises, and little ability for them to rip you off. I use prepaid AT&T (T-Mobile seems to have less coverage where I go) when I'm in the States, and the data is optional.
On prepaid it used to be that the available/required plans depended on the IMEI used to activate the SIM. So if you bring a crapphone and activate then switch the SIM into your iPhone you should be set and not need an upgrade. Not sure if that's still the case and they only periodically audit or what.