About a year ago I was able to go into AT&T online, Manage Account and simply disable the data plan.
Several options did not work anymore: email, appworld, Messenger, but I did not need them.
Web Browsing worked with Wi-Fi. Text, Voice calls and voice mail worked fine.
Recently I had to replace the SIM card, and, instantly a Data Plan was added to my account; the option "I do not need this feature" is not available anymore, but I switched to the $15, 200MB. AT&T forces You to have a data plan if you connect a BlackBerry to their network.
