If that sounds costly, you can alternately use a jailbreak iPhone utility to enable tethering. That’s going to cost pretty less if you are still on an unlimited data plan – just $1.99. TetherMe is a utility available on Cydia that will help you achieve this. What the application basically does is to tell the iPhone to allow tethering on any APN that is used for the iPhone data. Consequently, you will not be charged extra since it is seen as being accessed from the phone itself.
Additionally, the utility will also allow you to bypass signature checking. This is especially useful if you are unlocked to T-Mobile and want to enable MMS functionality. TetherMe works on iOS 3.X and iOS 4.X devices. Since this is technically not an app, you will not see a trace of it being available on the Home screen. You can instead change settings by navigating to Settings -> General -> Network and toggling the tethering settings.