The change will take place once Apple's new mobile operating system, iOS 6, launches for iPhones and iPads this fall. Only the iPhone 4S and third generation ("new") iPad will be able to use FaceTime over cellular networks. (The next iPhone, which will likely launch around the same time as iOS 6, should be able to make FaceTime calls over cellular too.)
Again, we have example of carriers nickel and diming you for data you already own. Right now AT&T charges $30 per month for 3 GB of data. That's data you bought. With your money. Just like you buy candy or books or video games or gasoline.