“But,” you say, “AT&T has yet to officially support data tethering on the iPhone OS 3.0.”
That may be true, but that hasn’t stopped the fearless iPhone community from tethering their iPhone 3G’s to AT&T’s data network for free! Thanks to the widely available iPhone 3.0 OS GM (gold master) firmware bearing the “7a341” designation, iPhone users can get their “tether on” without AT&T’s official blessing. We can’t tell you how or where to get your hands on the iPhone OS 3.0 GM, but a certain search engine (no, not Bing!) will probably do you right. Now that the iPhone OS 3.0 is officially official, tethering your iPhone 3G to your laptop/computer is even easier – as long as you have iPhone OS 3.0 installed on your iPhone 3G. 
[Update 2]
Here is an even easier, web-based method to enable Internet Tethering on your iPhone 3G (running iPhone OS 3.0). It’s the “ridiculously easy way to enable” iPhone tethering! Of course, the below method is quite simple and works as well.
Five steps to iPhone tethering goodness (make sure you’re upgraded to iPhone OS 3.0!):
- Download this carrier settings file
- Fire up Terminal on your Mac and type:
defaults write com.apple.iTunes carrier-testing -bool TRUE - Connect your iPhone to your Mac and fire up iTunes
- Option-Click the “Restore” button and select the .ipcc file within the .dmg (disk image) file
- Once the carrier file is installed, restart your iPhone for good measure
- Enjoy the new tethering option under “Settings”
With the iPhone OS 3.0 installed on your legit (using an offical iPhone carrier’s network) iPhone 3G, all it takes is a few button clicks and you’re off surfing AT&T (or your local carrier’s) data network through your iPhone 3G tether. We can confirm that the hack works on the iPhone 3G. Let’s hope it works just as well on the iPhone 3G S!
[Update]
Post updated with an easier tethering method!
[Update 2]
Here is an even easier, web-based method to enable Internet Tethering on your iPhone 3G (running iPhone OS 3.0). It’s the “ridiculously easy way to enable” iPhone tethering! Of course, the below method is quite simple and works as well.
[Via: MacMegaSite]