I just subscribed to the $20/mo, 10 MB/mo plan that Cingular offers. Unfortunately, while you can use your cell phone to subscribe, and they can give you all the information you need (although it's like pulling teeth to get Mac info...like so many others, they only officially support Windoze), you can't download the necessary modem script to enable the whole thing to work.
They recommend you download GPRS Script Generator, which requires that you pay the $10 shareware fee before you can generate the necessary script. Fortunately, a very quick search on Google turned up an article on how to configure the Mac for Cingular Motorola use. It provides a modem script that seems to work well.
I have a RAZR V3, and I modified the script to connect at 115200 bps (rather than the default 57600). It worked great!
This is all temporary, of course, 'till I get my 3G card. We'll see how smoothly that goes...