Update: Despite Tether's assertions, Apple has pulled this app from the store.
Update #2: Tether confirmed in a blog post that Apple pulled its iTether app because it could place a burden on a carrier's network. It appears that iTether may be gone for good. Apple reportedly "offered us (Tether) no way to remedy the solution."
BlackBerry users may remember TetherBerry, an app that lets you share your BlackBerry's cellular Internet connection with a computer. It caused a storm because it was one of the first third-party apps with tethering functionality that actually worked. The company behind this app is now disrupting the iOS world with iTether, the iOS version of their tethering app.
The iOS app lets you share your cellular Internet connection with your laptop or desktop via USB (WiFi or Bluetooth is not supported). It requires you to run an app on your iPhone and a companion app on your PC. The two apps bridge the connection between the phone and the computer. It doesn't require you to jailbreak your device, so everyone with a computer, a USB cable and an iPhone can use it.
I've used both the BlackBerry and the Android version and they worked great. I haven't had the chance to use the iOS version because Tether's servers are down and I can't download the Mac software needed to get it to work.
The developers behind iTether told MacRumors that the app is in the iOS App Store with Apple's blessing. "We were very clear with Apple what our app did. They asked us a bunch of questions and then approved us." If you really want the app, you should grab it fast as it may disappear once carriers start to complain to Apple. If you buy the app, be aware that carriers could detect that you're using it and shut you down.
The app, iTether, is available for US$14.99 from the iOS App Store.