

Despite including internet tethering as a feature in iPhone 3.0, U.S. AT&T customers have yet to have the feature enabled. Engadget got a comment from AT&T about the absent feature. The response cites high traffic usage that could hamper network performance:
Tethering is the process of using your iPhone as a wireless internet modem for your other devices such as your laptop.iPhone tethering has the potential to exponentially increase traffic, and we need to ensure that we're able to deliver excellent performance for the feature over and above the increases in data traffic we're already seeing before we will offer the feature.
Article Link: AT&T Continues to Delay U.S. iPhone Tethering Due to Concerns About Network Performance