Here you have an extremely easy way to have tethering enable on your iPhone 3G and 3GS without having to pay the expensive AT&T, O2 or any other network since this method is accepted in many countries, just by visiting a site on your iPhone’s Safari. No jailbreaking needed. just type the following address in Safari:

http://help.benm.at/help.php

Then scroll down to the Tethering & Internet Settings, then choosing your country and provider. This works for both AT&T and T-Mobile, and will let you install the appropriate configuration.

Now go to your settings and enable tethering. Check the video walkthrough bellow to help you configure tethering.