First, you’ll need to download this carrier update. Next up, open Terminal and type:
defaults write com.apple.iTunes carrier-testing -bool TRUE
Open iTunes, option-click the restore button and select the ipcc file in the disk image. Once the update completes, you should see Internet Tethering within your network settings.
Of course, this is not supported by AT&T. If they notice a big increase in your bandwidth, it could result in removal of service or unexpected charges or fees.
MacMegaSite via Crunchgear