According to TechFlash, this is what's going on:
[A]n AT&T spokeswoman told us via email today that it won't be possible to tether the iPhone to the iPad to share Internet access with the Apple slate, citing the fact that the iPad doesn't have any USB ports to enable a wired connection between the devices.
It's odd that the spokeswoman decided to specifically cite the impossibility of a wired connection when based on the iPhone 4.0 beta menus, iPhone tethering will be done through either USB or Bluetooth. So, does that mean that AT&T will somehow block our iPads from taking advantage of the Bluetooth tethering option? [Tech Flash]
Update 1: Looks like AT&T doesn't want to take the blame for this one. This is what we were told:
You'll need to speak with Apple. There is no AT&T policy around tethering and the iPad.
Update 2: Some clarification added regarding TechFlash's quote and AT&T's response.