From the article: "So far, we don't know of anyone who has received AT&T's e-mail in error. Did any of you receive the message, and if so what are you planning to do?"
Very interesting stuff. $30 for unlimited without tethering or $45 for 4GB of data and tethering is the choice some jailbreakers are seemingly going to have to make very soon.
Only AT&T knows how they are doing they detection and they aren't telling. So far it seems that no one who isn't tethering via jailbreak has got the notice, so at least AT&T's method is apparently accurate.