AT&T says it's unlimited but there is actually a cap that is just there to ensure that you don't find a way to bog down their network. I believe it's a 6GB cap? I'm not sure, but you shouldn't worry since it is pretty difficult to reach it even if you surf non-stop on the iPhone.
If you do, however, use a tethering on your phone to browse on another computer using your phone's connection then you may possibly go over.
And even if you do find away to go egregiously over your monthly allowance only using your iPhone, they probably won't charge you any fee. It's just so AT&T has the option of terminating the contract.