Tethering on 3G / 3GS iPhone

You obviously need iPhone OS 3.0 first, or else none of this is ever going to work.
It is dead easy, use your iPhone Safari to go to
http://help.benm.at/, choose your carrier and install the profile.
Then go into Settings / General / Network / Internet Tethering and switch it on.
Your iPhone will go a pretty blue colour when it starts tethering.
On a Mac, just plug the iPhone in, or connect with Bluetooth and it will automatically detect the new network interface.
On a Windows PC, you need to download the “
PDANet Desktop Client” to use it.

For more information, go
here.

Update 2009-06-18: The guys at Benm.at set the APN wrong in their settings file, their one will work but it will be trivial for O2 to find you have installed the file. If you use my version instead, then they won’t see any change in your APN settings. Point your iPhone at this page and follow this link to install my version.