Tethering
Jailbroken iPhones typically use the same tethering technique as a standard iPhone, the one thats already present in iOS. This method exposes tethering activity quite readily, because the iPhone, when in tethering mode, 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.
Some tethering applications for iOS make use of alternative methods and route tethered traffic through the phones normal data APN, but by and large, most jailbreakers stick with the stock application because its easy to use and doesnt require any complicated setup. In fact, many iPhone users jailbreak for the sole purpose of avoiding AT&Ts tethering fees (for why, see next section). These are the people AT&Ts is going after.
So it appears, apps that use the special data APN get noticed by AT&Ts systems, hence the strong-arm tactics. Popular jailbreak app MyWi works just as described by AndroidPolice using the tethering APN. What we need is an app that uses the same APN as your iPhone does for its own data. According to the iPhoneDownloadBlog, that app is PdaNet.