Great thread. PdaNet is definitely the way to go (as a couple of posters have suggested). Here's how to get it all working:
Follow this guide to Jailbreak your phone: http://www.iclarified.com/entry/index.php?enid=1881
Launch Cydia and search for "PdaNet" and install it on your iPhone
Create an ad-hoc network on your PC/Mac using this guide: http://www.junefabrics.com/iphone/faq.php#wifipc
Join your newly created ad-hoc network with your iPhone
Browse the web with your Carputer
I used this exact process (with a few Google searching in between) yesterday and it works great. PdaNet now works with Wifi (Apple/PC) and USB (PC) tethering.