AT&T’s leading wireless executive said yesterday that the carrier is working with Apple to devise a way for the popular iPhone device to be used as an ad hoc cellular modem when connected to a laptop.

The company expects to release a sanctioned tethering solution “soon,” said AT&T Mobility president and CEO, Ralph De La Vega, in an interview at the Web 2.0 Summit. AT&T already allows BlackBerry smartphone owners to use their devices as wireless modems, at an additional cost of $30 per month, but has not yet extended this feature to iPhone users.

Some users have managed to connect their iPhones to a laptop by downloading external applications such as PDAnet, which turns an iPhone device into the equivalent of a wireless router, accessible from any Mac or Windows-based laptop computer.

AT&T’s terms of service explicitly ban the use of such software, however, stating that “unlimited plans (except for Data Connect and BlackBerry tethered) 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 laptops, PCs, or other equipment for any purpose.”