We haven't tried this but someone has published a tutorial about how to enable 3G tethering in the iPhone OS 3.0. [Update: It works with AT&T]
Warning: ONLY DO THIS IF YOU KNOW WHAT YOU ARE DOING AND CAN ACCEPT THE RISK*
To get this to work with AT&T, follow these instructions:
1. Download this file. The image files contains a modifid ATT_US.ipcc file, which is a bundle that contains different property list XML files. These text files enable or disable functions in the iPhone.
2. Plug your iPhone with iPhone OS 3.0, and wait for it to connect to iTunes.
3. Option + Click on the "Check for updates" in the iPhone screen.
4. Select the ATT_US.ipcc file.
5. The iPhone's version of the file will update.
6. Once it restarts, go to Settings > General > Network and turn on tethering.
7. Enable the USB connection, but say no to the Bluetooth tethering.**
Done. Now connect your iPhone to the computer. The computer will automatically detect a network connection on the USB port. It just works.
Turn off your Wi-Fi or unplug your Ethernet to check how it works.
** Tethering over Bluetooth only works with older hardware, before the current revisions (Bluetooth to 3G tethering won't work in the unibody MacBooks.)
*WARNING: A reader reports that since installing this file this morning, he ran into some problems:
• Camera icon is gone in the messages app (doesn't matter, since there's no MMS capability yet).
• Voicemails are not coming in.
• There's no data on 3G bandwidth consumption in your online AT&T account. BEWARE: SINCE TETHERING IS NOT SUPPORTED BY AT&T, THERE'S THE POSSIBILITY THAT THEY MAY BE CHARGING YOU FOR USING IT
Click to view[File from joachimbeam—Thanks J.]