From The Apple Wiki

Tethering, in the case of the iPhone and 3G-capable iPads, is the act of using a proxy and a few other tools to make use of the 3G/EDGE connection from your computer or laptop. There are multiple methods of doing this, primarily editing the APN settings in a Carrier Bundle. As of iOS 3.1, the APN settings are signed by Apple to prevent editing, but this signature check can be disabled by patching CommCenter. Tethering is also made possible with applications on sale in the Cydia Store.