How jailbroken iPhone users can avoid being caught tethering by AT&T
AT&T actually has a very simple way to track down unauthorized tethering by iPhone users, because most tethering options use the same method as the official tethering, which routes your Internet traffic through an alternate APN (AT&T access point/router). This alternate APN is designed specifically to identify traffic as tethered, so AT&T can keep track of data usage and billing. Since many jailbreakers use the stock iPhone options for tethering, this makes it very easy for AT&T to track unauthorized usage and contact those in violation of their policies.
Of course, you'll need a jailbroken iPhone to take advantage of the app. Be warned that there are always risks associated with jailbreaking, so be sure you know what you're doing.
PdaNet is also available for Android, BlackBerry, and other mobile platforms.
source: Redmond Pie