Now that Geohot has released the blackra1n RC3 jailbreaking tool, activating tethering on an iPhone 3G or 3GS is a snap. Many international users already have tethering, meaning they can share their iPhone data connection with a computer. AT&T does not offer this option in the US.
Fortunately it's never been simpler to tether if your iPhone is running the latest firmware (3.1.2). Follow these steps to activate tethering, just keep in mind you are breaking all kinds of agreements with AT&T and Apple in the process.