I'm not sure how long this has been around, but when my coworker Todd was getting a new Cingular phone (the super-nice Moto v400), he discovered that Cingular is offering an all you can eat data plan similar to what T-Mobile has been offering for a while now.
For $19.99 a month, the MEdia Works plan includes 1500 (!) text messages, 200 MMS, and unlimited "Wireless Internet Express" (logos of the services available on their WAP portal above, though the plan allows direct data as well). Wow. Since I'm paying similar amounts at AT&T Wireless for 8MB of data a month, I think that's pretty damn amazing.
The idea, of course, was that AT&T had nationwide EDGE and i-Mode so I was going to be cool and super-advanced, but since the Nokia 6620 is Missing In Action, this has not been the case and I'm stuck paying the same for a lot, lot less. Let's hope when the merger comes through that this Cingular pricing stays.