Oh, I'm sure given the full weight of AT&T, all tetherers could be found and fined, but given at&t's lack of interest, I don't see anything more than the current half-hearted profiling coming into play.
Currently, AT&T just slaps the 4GB+tether package onto any 3G iPad that sucks down more than 4-5GB in any one month.
I know several folks who got slammed with it for non-tethered heavy use, and others who tether constantly without a word from the masters.
Razorback - Arch-6430U - i7-3687U - 16GB - Intel HD4000
Rocinante - Ubuntu-E5530 - i5-3230M - 16GB - Intel HD4000
Donnager - Arch-M4800 - i7-4800MQ - 32GB - Intel HD4600/Quadro K2100M
Vargr - Arch-Vargr - i7-4790K - 32GB - Intel HD4600/GeForce GTX 970