PDAnet is a software. taken from their website: "The latest version turns your iPhone (any version) 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."
Cool, I just used the carrier file that enabled the built in tethering on mine, but very nice. I hear that AT&T will start to notice you after 4-5 gigs, so be weary!