Cost of delivering bits on Wifi vs cost on 3G
3G Data is _EXPENSIVE_.
Operators are selling it at up to 10 times under cost (more in Europe) to fill up unused capacity with the prices based on cost of backhaul dropping by a factor of more than 10 times in the next 3 years due to "technological advances". However, looking at the tech roadmap for backhaul the chances of this happening is roughly equivalent to seeing lucifer driving a snowplough to work.
The price differential between it and wifi is probably around 50+ times per bit.
Add to that the fact that Apple iPhone defaults to WiFi when available and ATT has a valueable tool to cut its costs compared to EU operators. They will be idiots not to use it.