If Cingular is working for you, then great. They have a data plan that is 49.99 ( I believe without checking) that will give you data only, no voice.
Cingular's personal unlimited BIS (Blackberry Internet Service) plan without voice is $34.99. If you have a voice plan, the unlimited data is $29.99. You should go to the Cingular store and get it from them. The Customer Service Reps will try to tell you its 44.99 or some other ridiculous number.
You have to have a BB data plan to have email and Internet access.