Web developer Jake Marsh made a nice guide on how to to activate internet tethering on the iPhone 3.0. The guide is only for those who live in US and with an AT&T contract.
» iPhone 3.0: How to enable Internet Tethering

iPhone developer Steve Troughton managed to activate the tethering preferences while playing with the new iPhone OS. He was also able to actually tether his iPhone 3G to his laptop over USB. Based on the preference panel, it appears that you will be able to tether via USB or Bluetooth.
» USB Tethering working with iPhone 3.0

In a recent interview, Ralph De La Vega, AT&T Mobility CEO, admitted that an official tethering plan which allow you of using the iPhone as a 3G modem for a laptop is on the way “soon”.
» AT&T Official tethering plan coming soon??