It's not phone or plan specific: when your Iphone connects to the tower, it does so through the EDGE or 3G radio. The tower transmits back the data to your phone through its appropriate transmitter. AT&T keeps track of how and the amount of data transmitted. Trust me, AT&T is much too anal about making money too not keep track of who is recieving data and how. There is no way they'll let you recieve 3G data without paying for it. It's how they make money.
AT&T is much too concerned with making money to let people suck 3G bandwidth without paying for it.
Are they monitoring on a second by second basis, no. But you can bet it will be caught at the next billing cycle. You need to remember that the 2G and 3G networks are seperate technologies.
It's cool if you don't beleive me, but you'll understand after your first full billing cycle. Best case is they'll just add back in the $10 that you agreed to pay in the contract you signed. Worst case could be much, much more expensive depending on how much data you used over 3G