With iPhone software version 3.0 Apple has allowed for tethering, unfortunately if you live in the US and have AT&T it’s not working because AT&T hasn’t figured out how to charge you for it yet. Why wait for them to figure it out? Here’s how you can do it on your own:
1. Load up Terminal, and enter this (all on one line)
defaults write com.apple.iTunes carrier-testing -bool TRUE
2. Download this disk file and mount it.
3. With your iPhone connected to your computer, go to iTunes and option-click “restore” then select the disk image you just mounted and the file on it. If the first time you try this you can’t select it, just try again as some people have needed to do it twice.
That’s it! Now just enable Teathering by turning it on at iPhone/settings/general/network/Internet Tethering and you are good to go. Enjoy!