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.
Behemoth - Plex server
Galactica - FreeNAS
Pegasus - R710 ESX host
fsociety - MBP 15
Rocinante - E5530 testing machine
Saratoga, Ticonderoga, BunkerHill - the Hydra desktop PC-thing.