So if you haven't already discovered tethering is working on iPhone firmware 3.0 (even for AT&T). You can enable AT&T tethering via a simple .ipcc (download AT&T.ipcc) update via iTunes OR you can make it happen in an even easier manner via Safari on your iPhone!
BenM.at is hosting TONS of iPhone tethering configuration files for many many networks across various countries.
Simply browse to help.BenM.at/help.php via Safari on your iPhone
Select Install Now
That's it, Now just browse to Settings->General->Network->Internet Tethering->On and you have tethering.
Note: at the time of this writing tethering is still working for AT&T even though they said it would not work. Here's hoping that AT&T doesn't prevent this overnight!
thx to iClarified for pointing out BenM.at