AT&T is keeping the pressure on their grandfathered unlimited data plan customers.  Earlier today, AT&T confirmed rumors they are prepared to revoke customers’ unlimited usage plans if they are caught tethering their phones without an authorized tethering and mobile hotspot plan purchased directly from the company.  BGR quotes an AT&T spokesperson:

Earlier this year, we began sending letters, emails, and text messages to a small number of smartphone customers who use their devices for tethering but aren’t on our required tethering plan. Our goal here is fairness for all of our customers. (This impacts a only small percentage of our smartphone customer base.)

The letters outline three choices:

1. Stop tethering and keep their current plan (including grandfathered unlimited plan)
2. Proactively call AT&T or visit our stores and move to the required tethering plan
3. Do nothing and we’ll go ahead and add the tethering plan on their behalf — after the date noted in their customer notification.

An AT&T customer told 9to5 Mac he was threatened with the unilateral loss of his unlimited data plan if he was still unofficially tethering his phone after Aug. 11th, a date AT&T has since said may not be everyone’s “cutoff date”:

I was just informed that as of Thursday, August 11th, if you use MyWi or any tethering on the phone or using the phone as a modem, AT&T will automatically change your unlimited plan to a 2GB tethering plan for 45 dollars without the customer’s consent. This is for those who received emails or texts about the use of tethering without an AT&T tethering plan.

It’s clear AT&T is going hard line on their unlimited data plan customers, first sending notice they will throttle the speeds of any customer on an unlimited plan deemed a “heavy user,” and now threatening to terminate unlimited usage plans for customers who violate the company’s tethering terms and conditions.