Despite offering tethering for laptops, AT&T will not allow you to tether your iPad to your iPhone.
An AT&T spokesperson told TechFlash “it won’t be possible to tether the iPhone to the iPad to share Internet access.”
Tethering your laptop to your iPhone — an option built into the upcoming iPhone 4.0 OS — will be possible via USB or Bluetooth. But the iPad’s Bluetooth profile for tethering is not enabled.
TechFlash: AT&T: No iPhone-iPad tethering