One developer was " hacking around " with some carrier bundle files and iPhone 3.0 and somehow managed to activate the tethering preferences. Not only that, 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.

Tethering is a feature that allows you to share your iPhone's internet connection with your laptop, but has not been an allowed feature up until iPhone 3.0. Even with iPhone 3.0, mobile carriers generally charge an extra fee in the U.S. to allow tethering access.

Apple admitted that they had built in tethering support into the 3.0 release but had not yet coordinated with mobile carriers.