I did this about a month ago. My iP4 is chugging along with unlimited data.
Got a 2g iPhone from craigslist for $100. Did the trick.
Blame the FCC, not AT&T. FCC restricts wireless spectrum licenses. Also, the spectrum isn't infinite to support you downloading the latest cutest cat video from Youtube in HD while another 400+ people want to do the same thing on single tower...