Techs & Trends
I know for a fact that if your sim card is in a data phone, at&t DOES know and WILL charge you the $30 data plan. They will send you a text message before hand telling you this, whether you choose to ignore the message is your problem. However if you were using a data phone before Sept 09 without a data plan, you have nothing to worry about unless you change phone and/or sim card. The only way to take this data charge off of your plan is by getting your sim card into a non data phone. Again, your carrier DOES know what phone your using.
Now, what another person said about Tech support requiring proper phone information, this is correct. An early part of nearly every troubleshooting process we have is to verify that the phone in the system matches what the customer is using; if not, we must register the serial number, and we must place them on a compatible plan. A lot of tech ...
My understanding is that the only time that you are required to have data is when you are getting a free or discounted smartphone on a contract plan. T-mo also has some smartphone like the Nokia nuron and the Nokia Mode that don't require the web plan and only cost $10/mo if you do use it.