I will tell you what works for me, though there are the following differences, so YMMV.
--I'm on Tmobile, not AT&T
--I'm on an N800 with Chinook

First, your phone does showup in the control panel as a bluetooth phone, right? Control Panel -- Bluetooth -- Devices -- Edit -- Device Type Phone

Then, you should add .com to the cingular access point name when defining the Connection setup. My Tmobile access point name is wap.voicestream.com

Next, maybe, but probably not important, you have *99#1 -- that uses the first profile built into your phone. I don't have multiple profiles, so I only entered *99# into "Dial-up number".

Allen