if you dont have tethering, they will bill you. if you have the old $30 it will change to $45 for 4gb.
Fortunately, I am on the grandfathered Unlimited plan... However, they still try and bite those of in that plan to a tethering plan which puts us on a 2GB data package with Tethering.
I spoke with a few customer service reps and those that pull mass amounts of data by trying to run their whole house off the tethering or trying to torrent/game off of it, yes, they are going to get a nasty letter.
Those that use it whole on a bus to connect to work or just surf the web are not going to get a letter. Same with those that use the tethering to let their tablets on the net.
It's all about being smart. I have been tethering on AT&T for years. The most I do is check emails from my laptop and periodically need to remote into some servers to work, over all, it's about being smart on usage.
It's the few bad apples in the bunch that screw things up for everyone else.