Avoid data plan being added onto AT&T iPhone 3GS
You'll have to change the IMEI to avoid the charge. There is simply no other way. As soon a phone connects to a network, the IMEI is registered and that's what carriers like AT&amp;T use to determine if your device is a smartphone.

You can use ziphone to change the number but there's a lot of hurdles and provisos. As far as I know, that's the only JB program capable of doing it. Instructions are outline here: http://m.iclarified.com/entry/index.php?enid=657

The Motorolla RAZR's IMEI: 358472042445412

Another alternative is to go to T-Mobile which don't have the Draconian charge.