You will be forced to get the iPhone data plan if you buy the iPhone from AT&T (which is the suggested way to do it).
If you buy the iPhone somewhere else, you're lucky, extremely lucky if it's unlocked so that you can use it on AT&T.
Worst scenario? Buy the iPhone from AT&T and ask them to switch to an iPhone data plan. Since you're not ending your service, they should do it with no problem.