Just did a post on this at my iPad site. Every time you tap to view your cellular data account, cellular data is turned on automatically - and left on - without any sort of heads-up. I think is badly implemented and just plain wrong. Here's my article on this:
http://ipadinsight.com/ipad/why-does-att-automatically-turn-on-cellular-data-on-the-ipad-without-telling-you/
http://ipadinsight.com/ipad/why-does-att-automatically-turn-on-cellular-data-on-the-ipad-without-telling-you/