iPhone owners have been looking forward to the day when they will be able to use their phone as a modem, known as "tethering." In fact, this feature is an integral feature of the iPhone 3.0 firmware, and it is already available for some overseas carriers. However, AT&T has not yet allowed its American users to do so, though there has been hope that this feature would become available later this year.
Unfortunately AT&T does not appear to be ready to do so anytime soon. AT&T’s spokesman said "For tethering, we need to do some additional fine tuning to our systems and networks so that we do deliver a great experience." However, some believe that AT&T is actually concerned that tethering could be too demanding on its bandwidth. Stay tuned to see when AT&T will finally make tethering available to its iPhone customers here in the U.S.
Source: "iPhone tethering must wait, says AT&T" published at MacNN.