...For those that haven't tried this yet, keep in mind that you cannot recieve a phone call while your phone is being used as a modem.....
To be precise, while your phone is being used like a "Dial Up Modem" (i.e. CSD or "Circuit Switched Data") you can't receive or place voice calls & your minutes are charged for all carriers as far I know. Some let you use night & weekend minutes (t-mo), some don't (Cingular's new policy).
However, if your phone is using GPRS (and, I suppose, EDGE) you can use the internet and voice features of your phone at the same time. The technology is "Packet Switched" just like DSL.
GPRS charges are for the amount of data, rather than the number of minutes. This is an important feature for most of us. I want to be hooked up all the time but most of the time I'm not actually using the internet at all; I'm reading or writing or doing nothing.
Cingular and T-Mobile use GPRS technology & also support CSD. Other providers may do so as well.
Dave