AT&T can certainly detect if you're tethering.
Last week, T-mobile started blocking tethering on some customers' phones.
The detection software technology is there... its a question of whether AT&T wants to spend the money to implement it
agreed. it's easy for them to detect high data usage and do something about it. this is called "data hog" in the industry. they can lower the priority of the offending users so that they get less piece of the bandwidth when the network is loaded.