I'm going to jump in here and try to clear up the confusion....feel free to tell me to stay off the Blackberry boards until I own one....
I have a "regular" phone, and I use MOBILE IM - I have no data plan, and do not get charged data for using MOBILE IM. It connects using my AIM or Yahoo screen name, and I can send messages to and from users on a computer - for me it all counts as text, NOT data. I am not using the browser to log in to AIM or YIM, I am using Mobile IM (that came installed on the phone) and all my messages are TEXT.
On the Blackberry, there are apps for this type of communication, as Doc said - and if you are using the Blackberry apps, you use DATA.