If you are running iPhone 3G on 3.1.2, a new cydia patch will bring you tethering. This commcenter patch allows your iPhone 3G to be used as a normal tethering model for your PC or Mac. You need to reboot the iPhone 3G after install.
Warning: It might be against your carrier contract to use tethering.
Via iFoneguide on Cydia – Search for ‘Tethering patch 3G‘