Now that you’ve got your iPhone 3GS or iPhone 3G with iPhone OS 3.0 you just might want to have a go at tethering. AT&T is still deciding just how much a tethering data plan would cost on the iPhone and even how it would be structures so currently doesn’t allow tethering.
However, iphonefaq has come up with a way of allowing tethering as long as you have iPhone OS 3.0 and a computer running iTunes 8.2 on Mac OS X and using USB or Bluetooth via your iPhone to connect your computer to the AT&T data network.
First enter defaults write com.apple.iTunes carrier-testing -bool TRUE…next download and mount iphonetetheringfile.dmg… open iTunes and connect iPhone…hold down the options key, click restore button in iTunes, choose file ATT_US.ipcc from the open iPhone Tethering File disk image and click OK and settings will update the iPhone…lastly on the iPhone go to Settings -> General -> Network -> Internet tethering and touch ON.