Cingular just recently added an unlimited data option to their prepaid system, for ~$20/month

You need to be on one of their monthly minimum plans - so you'll be giving them ~$30/month for the basic plan, and then $20 extra for data.

There is no set up charge of any kind, although you will have to purchase a phone from somewhere. I tested the data service out on my 770, and it Just Worked(tm).

Note that you aren't supposed to tether your phone to a laptop or other device, and Cingular may do bad things if you try. The rumors I've heard suggest that 3-5 megabytes a day is the limit before they notice you.