They can't charge you for something you have asked them to disable.
Not completelly true.
Its not if they cant but if they will or when they will IMO.
I know people that have been using iphone 3G models for years on AT&T without data and with data blocked. (fake APN)
I know one person that has 2 3GS, 2 3G and 1 2G iphones all in his family plan without any data. Just calls and texts.
My wife so far has been using her 3G model for 5 months without data with no probs.
All they did was install the fake apn thru the iphones mobile safari browser www.unlockit.co.nz
No visual voicemail or MMS but everything else works fine. Data only thru wifi connections.