Missing the point. You can keep the unlimited plan as long as you never drop it. Once you drop it its gone. That is effectively the same as having a contract and changes the entire 3G concept of only paying for data when you need it. Were ATT to require a contract from the start it would be far smarter to have bought a WiFi model and a Verizon or Sprint MiFi.
Bottom line is if ATT couldn't afford that concept they should not have offered it in the first place. That is the point. When Jobs announced the plans in Jan there was an implied promise that those would be the plans at least until the next model. ATT broke that promise. Maybe they are under no legal obligation, but its bad faith nonetheless, and will burn them in the end.