Usually, Apple doesn’t allow apps in the App Store that allow users to unlimited tether for a one time fee, but it looks like Tether.com’s app has somehow been approved into the App Store. Their application is called “iTether“, which allows users to share their iPhone’s internet connection with their Mac or PC computer over USB. iTether costs $14.99 and is available via the App Store.
Tether offers applications for the Mac and PC; so the app should work on both platforms. Running the apps on your iPhone and your Mac allows your Mac to use your iPhone’s internet connection over a USB cable. With iPhone’s built-in tethering feature, users have the option to tether via Wi-Fi or Bluetooth, but this app currently does not support either. Apple usually does not allow such applications to become available on the App Store due to agreements with carriers.
Nobody knows how long this app will be available via the App Store; so, grab it quick! Also, it should be noted that AT&T is cracking down on unofficial iPhone tethering.
Update: The app has now been pulled.