Want a fairly easy way to be able to tether via WiFi or USB? THen here ya go:
First you need to have your iPhone jailbroken (if it's not already).
Then in Cydia you need to add the ifoneguide repository in the "more sources" section. Then still in cydia you need search for "tether" and then download the patch for either 3G or 3GS (whichever one you have).
Then need to go to http://help.benm.at/usa.php on your phone and choose your carrier. It will install a profile. Reboot phone and then in settings - general - network will be "internet tethering"
When you turn on tethering, it will give you the option to use USB or WiFi.