In March, AT&T began cracking down on customers it believed were jailbreaking their iPhones and using "unauthorized" apps to tether data for no extra fee. Text messages were sent out to suspected customers with an ultimatum: Stop tethering, or we'll automatically enroll you in our $45 DataPro plan.  

The folks over at Android Police point out that jailbroken iPhones are easy to pinpoint when tethered, because many tethering apps use the same technique as the one inherent in iOS. When tethered, the iPhone "sends traffic through an alternate APN (AT&T access point/router) for the express purpose of identifying the traffic as tethered data. This makes it extremely easy for AT&T to identify whether or not an iOS device is utilizing tethering, and just how much of their data is consumed via tethering."

The customers AT&T is likely targeting most are those with the grandfathered unlimited data plans. They can tether their hearts out, without worrying about going over their allotted data cap.

IPhoneDownloadBlog has a list of iPhone tethering apps. MyWi, the most popular one, uses the same APN as the iPhone's standard tethering system, so it is not recommended if you want to go undetected. Two other alternatives are PdaNet -- which offers a "hide" function -- and TetherMe.

However, some users in the comments section of iPhoneDownloadBlog are saying that even with the updated versions of these tethering apps with hiding functionality enabled can’t escape AT&T’s wrath.

“I used the new PDANET since the day it was released a few weeks ago,” wrote one commenter. “Shortly after and consistent with 4.3 upgrade I received an email from ATT.”

So perhaps no one is safe anymore when it comes to unauthorized tethering on the iPhone.