The following is a major, MAJOR caveat for Cingular subscribers to the $20 unlimited data plan.
When you receive a unbranded Nokia phone, you may obtain the Internet connection settings from Nokia's website or from the settings wizard on the phone itself. In both cases, Nokia sends several settings to you OTA. One of them is called Cingular Internet, and it uses the server isp.cingular. Do not use this connection, or you will be billed according to Cingular's pay-per-kilobyte plan at 1 cent per KB.
Delete this access point and use the one called Internet-GPRS. It connects to the server wap.cingular. If you have the $20 plan, you can download to your heart's content and not incur additional charges.
Case in point: I'm a heavy-duty Internet user. When I received my new N80 a couple of months ago, I immediately downloaded Nokia's Cingular settings and started browsing the web. A month later, I received a bill for about $800 worth of data usage.
After recovering from the shock, I investigated my connection settings, and found that I was using the isp.cingular server. To my great relief, the techs at Cingular gave me a break and reduced the data charges to $20.
Just be sure your Internet access point is wap.cingular, not isp.cingular.
Cheers,