AT&T does not currently allow tethering on the iPhone even though Apple's iPhone OS 3.0 includes the feature. The company is still deciding what a tethering data plan for iPhone would cost and how it would be structured.
Tired of waiting? If you have an iPhone 3G or 3GS running OS 3.0 and a computer running iTunes 8.2 on Mac OS X here's how to activate tethering. You will be able to connect your computer to the AT&T data network using USB or Bluetooth via your iPhone.
1. Enter the following command into Terminal:
defaults write com.apple.iTunes carrier-testing -bool TRUE2. Download and mount the file iphonetetheringfile.dmg on this post at 9 to 5 Mac.
3. Open iTunes and connect your iPhone.
4. Hold down the option key and click the restore button in iTunes. Choose the file ATT_US.ipcc from the open iPhone Tethering File disk image and click OK. The settings will update on your iPhone.
5. On the iPhone, go to Settings -> General -> Network -> Internet tethering and touch ON.
Take it easy on the bandwidth consumption. You are now successfully in violation of your AT&T contract terms, and might be charged exorbitant fees or have your mobile account suspended for an indefinite period.