Woohoo…tethering enabled on the iPhone finally. Sucks ATT has this disabled but with a few clicks and 5 minutes you can turn it on.
DISCLAIMER: THE FOLLOWING STEPS MAY VOID YOUR IPHONES WARRANTY OR VIOLATE YOUR AT&T TERMS OF SERVICE!
Mac OS X:
1.    Close iTunes if it is open
2.    Start Terminal (Applications > Utilities)
3.    Run the following command: defaults write com.apple.iTunes carrier-testing -bool TRUE
Windows 2000/XP/Vista (32-bit):
1.    Go to Start -> Run
2.    type in “cmd” and hit enter
3.    Type/copy+paste this into the command console that comes up: ”C:\Program Files\iTunes\iTunes.exe” /setPrefInt carrier-testing 1
4.    Hit enter
Windows XP/Vista (64-bit):
1.    Go to Start -> Run
2.    type in “command” and hit enter
3.    Type/copy+paste this into the command console that comes up: ”C:\Program Files (x86)\iTunes\iTunes.exe” /setPrefInt carrier-testing 1
4.    Hit enter
After that, the steps become a lot more simple.
1. Download this file (for AT&T) and mount the disk image (thanks Gizmodo).
2. Option-click on “restore” in your iTunes with your iPhone attached.  Browse to the opened disk image file and hit OK.  It will update phone settings for a few seconds.
3. You have tethering.  Now go into your iPhone/settings/general/network/Internet Tethering and turn it on.

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