One of the new features in iPhone OS 3.0 is the internet tethering that allow you to share 3G network to your computer through bluetooth or USB cable. However, the function may be disable per carrier setting. For example, currently AT&T in US disallows internet tethering. However we can re-enable it by installing a customized network configuration file to your iPhone. The trick don't need your iPhone to be jailbreak. You must need to do some steps in the Mobile Safari of your iPhone. Note that only iPhone 3G and iPhone 3GS support network tethering and the original iPhone 2G don't support it.

1. Open safari in your iPhone, and visit http://help.benm.at/help.php
   
2. Scroll down and you will see the network tethering section, click the "Download", it contains almost all setting of network carriers in the world, just select the carrier that your are using
   
   
   
3. Then the website will create a customized network configuration profile for your and allow your to install to your iPhone
   
   
4. After the profile is installed, you should able to see the "Network Tethering" option in "Settings->General->Network"