I was able to get my iPhone 3g to tether to my mac! Currently I only can use firefox, but that is still a huge plus.
Warning... try at your own risk. Not my fault if you mess something up!
First off you need a jailbroken iphone with 2.0 firmware.
Use Cydia to download and install SOCKS Relay and OpenSSH
Then you need to try and follow the following guide... thats what I did! good luck!
http://www.zdziarski.com/papers/tethering.txt
I'll try to answer questions as best I can, but honestly I do not have that much experience :/
Tip:
-change your password once you are ssh'd into the phone, anyone on your network who knows the default 'alpine' password will have root access to your phone!
Barold