Alright, here is how to enable free internet tethering on iPhone 3G and 3Gs running 3.1.2 firmware. Jailbreak your device using The newest version of blackra1n, (
blackra1n) then open the blackra1n icon on your iPhone and install Rock App. Once the installation is complete (which will require a soft restart) search for "Enable tethering" in Rock App. There are two different versions of this tweak available, one for iPhone 3g and another for iPhone 3gs. Download and install the one that corresponds to your device and it will require a full restart of your phone (which it will prompt for), upon respringing, internet tethering will be available in Settings>General>Network>Internet Tethering. Turn tethering on and plug your device into your computer and enjoy free internet tethering through USB and Bluetooth. Unlike MyWi this doesn't cost a thing, and as far as I can tell my MMS and visual voice mail are still running perfectly. Happy tethering.