It works for a time
until ATT makes a random check on you and sees that your back to using your iPhone
then they add the data plan back on, and even have the right to charge you for all the months of data you "skipped out on" (it's in your contract, you agreed to allow them to do this)
there really is no reason for you to have an iPhone if you are unwilling to pay for data. That's the reason phones that aren't as expensive and don't require data plans exist. If you really like the internet/interface/functionality of an iPhone, your better off with a normal phone and an iPod touch.
however, it is still a possibility that your idea MAY work. Just don't be upset/surprised if you get a huge bill for all those months you didn't have the data plan. Also, if you do pull this off, be VERY careful not to EVER use data. It adds up fast, and suddenly you could be "saving" 30 bucks, but spending thousands because you have no data plan.
personally, I think you'd be better off finding something you can cut out of your expenses. It's 30 bucks a month. I'm sure theres something else you do frequently that you can go without, and will save you 30 bucks a month by foregoing. But thats just my opinion. Maybe I'm wrong