JB'd, and ultasn0w'd on iOS 4.0. I can see the "Carrier" option with "AT&T" in the main Settings menu.

But when I go to General / Network, I see "Internet Tethering" with the swirly wait icon, and then it changes to "Setup Internet Tethering".

Do I need to insert a non-AT&T SIM first to let is enable tethering on AT&T? Or is there another hack to do that?