I have done something similar with my nokia n810... I have a motorola v325i on verizon that I tether to the n810 through bluetooth. The n810 lets you talk to the phone as a modem and you can set up a new dialup connection with the phone. I set the phone number to be the local line for one of the many free dialup providers that are easily findable.
Verizon lets you assign custom "any network" type numbers to your account so you can get unlimited minutes to those numbers any time of the day... I'm sure most of the other providers do something along those lines. I've tested it extensively and it always shows up on my bill as a normal phone call... they can't tell it's modem tones and not voices. So as long as I'm not using any minutes for the phone calls it's unlimited free internet anywhere there's cell signal.
I can also do the same thing by tethering the phone to my netbook, and the phone has a mini usb type b jack on the bottom which makes it very convenient for connecting to a host device via usb... no proprietary connector to deal with
I average about 4-5 KB/s (that's KibiBytes) per second so I guess that's about full 56k modem speed... for free... anywhere there's cell coverage... yeah it's sweet...
yeah it's slow
It works great to check the email, forums, wiki, or do some searching... that's about it. For those of you who have your fancy schmancy wireless broadband then you're probably looking for some other way to do the dialing-up part.
I'm curious to see if my phone will work as a modem for the pandora... if so *yeaahhh under breath and thumbs up*