There is no mention of a tethering price in the contract.
Don't you think this makes it even more likely it will be an add-on premium service? I do.
The contract says this:
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.
My guess is that they'll offer a new combo data plan that includes tethering for the iPhone and adds an extra charge.