This just in: AT&T is cutting the price of their unlimited voice and data plan for the iPhone to $99. The old price was $129. There is no word (yet) about whether they will cut the price on the other, not-unlimited plans.
Existing iPhone users can switch to the cheaper plan by going to AT&T’s website starting Monday January 18th, 2010. There is no fee for making the switch and the end date of your contract will not change. Basically, if your iPhone is on an unlimited plan, it’s a no-brainer. Change to the $99 plan.
There is always the chance that AT&T will do this automatically for you but I wouldn’t count on it.