Unlimited texts cost $20 with att. Voice plans start at $40 for 450minutes and go up from there. Data is $30 for consumers and $45 for business.
The iphone will follow the same pricing scheme as the rest of smarphone lineup on att. All the information is available on their site, not sure why have seen so much confusion on this board about iphone pricing.
Even though they will most likely force you to get the data plan at first, if you bitch enough, you could probably get that taken off. But be careful. Most of the apps on the phone require some kind of data usage. You don't want to be paying per use, because it would be worse than just paying the $30.