While AT&T is dragging its feet on official tethering support for iPhone customers, that doesn’t mean you can’t do it now with a relatively simple workaround involving a custom carrier settings file.
iTunes 8.2, which you will need for tethering to work (whether you’re on OS X or Windows), disabled the ability to load the custom carrier settings, however there is a trick to re-enable it. Sadly, this will only work for OS X users – we’ll keep you posted on a solution for Windows users.
1. Download this disk image containing the carrier settings, courtesy of Gizmodo.
2. Quit iTunes, and type the following command into a Terminal:
defaults write com.apple.iTunes carrier-testing -bool TRUE
3. Restart iTunes, with your iPhone plugged in via USB.
4. Hold the Option key, and click the Restore button.
5. Select the .ipcc file in the disk image you downloaded earlier.
6. On your iPhone, go to Settings > General > Network > Internet Tethering and switch it on.