AT&T has begun cracking down on smartphone customers—particularly iPhone users—who have been secretly tethering their smartphones to their laptops without paying for a tethering plan. The company sent text messages to the offending users, followed up with an e-mail, that says they've been identified as taking advantage the feature without paying up. If those users continue to tether and don't make changes to their accounts, AT&T will automatically begin charging for the DataPro tethering plan from March 27. Those who quit tethering before then won't have to upgrade their accounts.

"Many AT&T customers use their smartphones as a broadband connection for other devices, like laptops, netbooks or other smartphones—a practice commonly known as tethering. Tethering can be an efficient way for our customers to enjoy the benefits of AT&T's mobile broadband network and use more than one device to stay in touch with important people and information," reads the beginning of AT&T's e-mail.

"Our records show that you use this capability, but are not subscribed to our tethering plan... if we don't hear from you, we'll plan to automatically enroll you into DataPro 4GB after March 27, 2011. The new plan—whether you sign up on your own or we automatically enroll you—will replace your current smartphone data plan, including if you are on an unlimited data plan."

AT&T currently offers either a $15/month 250MB 3G data plan for smartphone users, or a $25/month 2GB plan. If customers want to tether the legit way, they must subscribe to the 2GB data plan and pay AT&T another $20 for the privilege of sharing their smartphone's 3G connection with a computer or hotspot, at which point the monthly data cap goes up to 4GB. iPhone users who jailbreak their devices have long been able to tether without paying anything extra to AT&T—it's one of the main reasons people jailbreak—making those iPhone users particularly susceptible to AT&T's latest crackdown.

As for how AT&T is identifying these users, it's possible the carrier is simply looking at data usage trends and making an educated guess. It's also possible, as pointed out by TUAW, that browser ID strings are giving tethering users away. Or, as a reddit commentor points out, AT&T may be noticing different-than-expected TTL (Time To Live) units on packets sent from a computer through a tethered smartphone. Either way, AT&T is keeping mum on exactly how it's detecting tethering users and claims the e-mails are only going out to a "small number of smartphone customers."

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?