In 10 steps:
- Update iTunes to 8.2 via Software Update
- Update your iPhone to the 3.0 release (out today - June 17th)
- Download this dmg and mount it: tethering file
- Enable hidden carrier testing option (in Terminal.app): defaults write com.apple.iTunes carrier-testing -bool TRUE
- Start up iTunes
- Go to you iPhone's summary page
- Option-Click the Restore button
- Select the ATT_US.ipcc file from your mounted dmg (step 3)
- Open Settings -> General -> Network -> Internet Tethering on your iPhone and switch it ON
- When your computer is connected via USB you will see a dialog to open network preferences. Open it and hit apply.
If you have problems with visual voicemail after this follow these steps (thanks to luigi for pointing this out):
- Connect your iPhone and fire up iTunes
- Option-click on the Restore button like you did before
- Navigate to [your-username]/Library/iTunes/iPhone Carrier Support
- Click on the file in there to reset your carrier settings