Came from a link at iPod Server dot com Read comments of some who tried it.

Easy iPhone Tethering for AT&T users
By Mike on June 17th, 2009

If your iPhone is running 3.0, itís very easy to enable internet tethering on AT&Tís network with no ugly hacks. First youíll need to download this carrier settings file from gizmodo.

Next, quit iTunes and enter the following command in terminal:

defaults write carrier-testing -bool TRUE

Contrary to many articles, you donít need to install a pre-release version of iTunes.

When you restart iTunes, connect your iPhone and youíll be able to option-click the Restore button to choose the .ipcc file in that disk image. Rather than doing a full restore of your iPhone, it will simply update the carrier settings. If the update is successful, the Network page under general settings will show an Internet Tethering option.

Have fun!

Thanks to AppleNova for the terminal trick.