It is stealing when you signed the terms and condition that the data is only for your iPhone usage.
OP: ATT knows that you are tethering but just won't do anything.
Not really. If he ran SlingPlayer 24\7 he'd use a hell of a lot more data than if he used the internet casually via tether. You automatically assume that everyone who tethers is setting up a torrent connection and downloading 24\7.
Bottom line, they don't know, they just see usage, but don't have the resources to look into what the traffic is, nor any legal grounds to prove you're running a DHCP server dishing out an IP since all the data is processed by your phone's IP... they'd rather deal with it by penalizing everyone for the couple that abuse it, and impose tiered data pricing.
The funny part is if AT&T included free tethering like many carriers do, I'd bet of all the customers who tether, 99% would use it, and 1% would abuse it, so again the 1% ruins it for everyone. At the end of the day, regardless of your feelings of the legality (it's illegal to tether under the contract: FACT) the bottom line is it's a wireless network, and in 2011 there's got to be an understanding that the network isn't rock solid, and you can't treat it like your 30 mbps home Cable\Fiber connection... there's just not enough bandwidth to go around.