AT&T's CEO Ralph De La Vega told Michael Arrington at the Web 2.0 Summit, that the company is working on a way fro the  iPhone serve as a tethered wireless modem for laptops soon.

Currently using the iPhone as a modem violates AT&T terms of service.  If modem service is available there will probably be an added monthly rate.

AT&T already offers tethering on some smartphones, including some
RIM Blackberry phones, for around $60 a month for unlimited data access. 

There's been a lot of conjecture of what will happen to network traffic if modeming is allowed. Many iPhone users already claim that AT&T's 3G is slow.