Phone plan for the whole family?

[Sxotty]

So engadget has an article up about AT&T sending warning text messages to iPhone users who are "illegally" tethering their handsets.

How exactly would AT&T know a handset is tethering? For example I have an app on my Symbian phone that allows me to create a wifi hotspot. It uses NAT. Granted, I'm not worried as I have a 2004 unlimited data plan that makes no disclaimers about not-tethering as that was a non-issue then.

Just curious how AT&T would know without snooping (thus violating wiretap laws).

Generally in the past they could tell by what applicaionts were getting data, not the data itself. So if the useragent said this is the firefox browser then they knew the user wasn't using a phone to get internet.