Some AT&T iPhone users, Jailbroken and tethering via MyWi, are getting text messages and e-mails from AT&T that imply they know they are tethering even though they aren't on an official tethering plan. One of our own writers, Andrew was one of those who received the message.

I use MyWi to tether as well but have not received anything yet. (I actually was just tethering today.) I'm not sure how they are detecting MyWi users but I can make some educated guesses.

  1. Since AT&T offers Personal Hotspot under iOS 4.3, they may have access to data they didn't before, and they're choosing to use it in a pretty crappy way. So if you're on iOS 4.2.1 still (almost all jailbreakers are) and they see you're generating traffic that looks like Personal Hotspot, they know you're doing so without a plan.
  2. Like Chris says below, browsers use the wap APN. Tethering uses the ISP APN. As such, larger data is expected through ISP not through WAP. So if AT&T sees a lot of WAP data it may raise a flag.
  3. As several people say in the comments and Twitter, if AT&T sees a TTL reduction that suggests a device like a laptop or iPad in between the connection, it may raise a flag.
  4. They're just going after heavy data users again and trying to bluff them into switching to a tethering plan and losing their unlimited data. Andrew tethers a heck of a lot more than me. I only consume a fraction of what he does. So that's a viable answer too.

Originally Jailbreakers used MyWi because AT&T took a year longer to offer iPhone tethering than international carriers. Since then they've used it to get around AT&T's ridiculous fees for tethering. (Tethering is free on some international carriers as well.)

Hit the jump to read the e-mail AT&T has been sending out as well as to let us know what you think! Anyone else been getting messages from AT&T? Anyone on iOS 4.3 using official Personal Hotspot? Any PDANet users?

UPDATED: AT&T spokesman Seth Bloom told Phone Scoop:

[ModMyi Forums and Phone Scoop]

Dear [Customer],

We've noticed your service plan may need updating.

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. To take advantage of this feature, we require that in addition to a data plan, you also have a tethering plan.

Our records show that you use this capability, but are not subscribed to our tethering plan.

If you would like to continue tethering, please log into your account online at Cell Phones and Cell Phone Plans - Wireless from AT&T, or call us at 1-888-860-6789 Monday - Friday, 7 a.m. - 9 p.m. CST or Saturday, 8 a.m. - 7 p.m. CST, by March 27, 2011 to sign up for DataPro 4GB for Smartphone Tethering.
Here are details on the plan:

DataPro 4GB for Smartphone Tethering
• $45 per month (this gives you 4GB in total, combining both your smartphone data plan for $25 and the tethering feature, $20)
• $10 per each additional GB thereafter, added automatically as needed
• Mobile Hotspot capabilities are included for compatible Smartphones

If we don't hear from you, we'll plan to automatically enroll you into DataPro 4GB afterMarch 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.

If you discontinue tethering, no changes to your current plan will be required.

It's easy to track your usage throughout the month so there are no bill surprises. For example, we send you free text messages when you reach 65, 90, and 100 percent of your plan's threshold. If you would like to monitor your account more closely, go towww.att.com/dataplans to learn about other ways to track your data usage.

As a reminder, our smartphone data plans also include unlimited usage of Wi-Fi at no additional charge. AT&T smartphone customers can use Wi-Fi at home or on-the-go at any one of our more than 23,000 U.S. hotspots already included in your data plan.

Thank you for bringing your account up to date. We appreciate the opportunity to continue to serve your mobile broadband needs.

Sincerely,

AT&T