Previewed at WWDC, launching in the fall.
AT&T CEO Confirms iPhone Tethering Coming "Soon"
Tethering is the process by which you allow the iPhone to act as a 3G modem for your laptop. This would allow you to share the iPhone's cellular connection with your laptop.
A 3rd party tethering option called NetShare briefly appeared in the App Store before it was removed by Apple. AT&T traditionally charges an additional $30/month for this tethering option on their other smartphones. MacRumors has separately heard that this tethering capability is expected to be built into the 2.2 firmware.