An app that has been a longtime favorite of the BlackBerry community, Tether, has been published to Apple's iOS App Store under the name iTether. It's a $14.99 app that works in conjunction with an app on your computer to provide tethering over USB. On a Mac, at least, Tether works by creating a software-based Ethernet port to communicate with the iTether app on your iPhone. We've tested it here and it works fairly decently, though you have to keep both the app on your desktop and iPhone open in order for tethering to work.
There's a fairly good chance that Apple will pull the app in short order, so if you're interested you should grab it quickly. One big caution: although the app works even if you don't have a proper tethering plan with your carrier, we do not know if iTether looks like phone data or tethered data to the network. In other words, as with tethering apps on other platforms, using it is no guarantee that you won't get a message from you carrier telling you to switch to a tethering plan.
You can find iTether at the iTunes source link below ...for now.