Officially, thethering is not an option with the iPhone 3.0 for most AT&T customers in the U.S. Well, there’s an easy tweak to enable this feature in a matter of seconds, the problem is that AT&T and Apple don't support this tweak so use at your own descretion. Oh, and there is no guarantee that this will work...

1. Download this carrier update file from Wired. Then extract it.

2. Launch the Terminal app and type the following command:

defaults write com.apple.iTunes carrier-testing -bool TRUE

 

3. Now launch iTunes. In the iPhone window,  hold the Option key while clicking Restore. Then select the ipcc file in the disk image you downloaded and click OK.

4. iTunes will update the carrier settings. Now under General settings, tap the Network tab and an internet tethering option should appear. From there on, you should be able to easily tether the iPhone via Bluetooth or USB.

NOTE: This hack appears to disable Visual Voicemail. Did you think it wouldn't come at a cost? There's a way to get it back... Read More @ Wired