While I can not speak to your model, I can speak generally, as I recently checked this out. When you tether a cell phone via USB to a laptop, it shows up as a USB modem. When Cingular says the Mac OS is not supported, they mean that their pretty GUI dialer is a Windows-only app. BUT...
All you have to do is have the data service added to your rate plan [a phone call can do it], get a USB cable & plug in your wireless data capable phone into a USB port. Assuming OS X sees it, all you have to do is manually create a dial-up networking session. How to set that up? What username & password to use? Guess what, their are complete instructions on Cingular's own Web site!! It really urks me the drones say OS X is not supported, when in fact it certainly can be done!! I was looking for the link but have to go. I found detailed instructions at the Cingular site, under the business side.
Good Luck!