If you’re employing any unofficial iPhone tethering methods to connect to AT&T’s network, prepare to receive a stern warning – and perhaps additional charges, from the telco giant.

As reported by OSXDaily.com, AT&T is now issuing warnings to customers unofficially tethering their smartphone devices to its network. But that’s not all. The company is also giving customers an ultimatum: Either update your service plan to include a tethering plan, or you will be automatically enrolled in the AT&T’s DataPro 4GB package that costs an additional $45 per month. Stop tethering now, adds the notice, and all will be forgiven with no changes made to your plan.

OSXDaily.com features the full letter that AT&T is sending to its iPhone customers who are suspected of unofficially tethering. Here’s an excerpt:

“Dear [Name of Account Holder],

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 www.wireless.att.com, 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.”