Yes, you need the data plan to run Yahoo Messenger, Google Talk, and a slew of others. Anything that uses the internet basically....(except for T-Mobile's OZ chatting app).
And Cingular has NO PLANS to ever enable it to work. You will not get the service book from them for this. Cingular does NOT CARE what the users want....
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Currently use: Torch 9800 (unless I cant get the WiFi to auto connect, then I am getting a Galaxy S Android)