PdaNet -- Use your iPhone as a Wireless Router for your PC/Mac
vowe nails it – this will finally make me jailbreak my iPhone:
PdaNet has been one of the most popular software for Windows Mobile phones and Palm OS phones. It is now ported to the iPhone! The 1.2 version turns your iPhone into a true WiFi router for your computer (MAC or PC), and allows your laptop to go online wirelessly through the 3G network on the iPhone.