From the TOS:
"Furthermore, plans(unless specifically designated for tethering usage) cannot be used for any applications that tether the device (through use of, including without limitation, connection kits, other phone/PDA-to computer accessories, BLUETOOTH® or any other wireless technology) to Personal Computers (including without limitation, laptops), or other equipment for any purpose."
The iPhone is not specifically designated for tethering.
The 5 GB limits people are putting out are for air cards and the plans that a poster posted above (thanks for somewhat settling where the soft cap limit rumors are coming from!). We know nothing about what AT&T will do for the AT&T. Anything before they tell us if pure speculation.