Verizon has followed AT&T’s lead and taken measures to block customers from tethering their iPhone’s data connections illegally. Customers have long been using jailbreak apps to avoid their carrier’s additional monthly fee for tethering, but this method may no longer work if you’re signed up to AT&T or Verizon.

AT&T is informing jailbreak tetherers on grandfathered unlimited data plans that either they stop tether or sign up for 2GB tethering plan. If you don’t abide, AT&T will sign you up for a 2GB tethering plan and kill your grandfathered unlimited data plan for good.

A thorny wicket. It’s probably not fair to congest AT&T’s networks by sucking up data from your iPhone to play World of Warcraft on your MacBook Pro without, you know, paying for it, but if you’re intent on doing so, there may be a way to avoid AT&T’s scrutiny, keep tethering through your jailbroken iPhonei, and keep your unlimited data plan, all at the same time.

Popular jailbreak tool PdaNet has just been updated to introduce a nice new feature that hides iPhone tethering usage from its carrier. The update comes as AT&T begins a crackdown on customers who illegally use their device’s data connection without paying for a tethering plan.

To identify illegal tethering, AT&T looks at the type of packets hitting its network; any packet that doesn’t come from an iPhone is checked and warnings are then sent to the offenders informing them they must cease their wrongdoing or be billed for their use.

PdaNet’s new feature works by masking those packets and making the carrier believe they’re coming from an iPhone, and that all of their subscribers are being good boys and girls.

We all gave up the ability to jailbreak and tether our iPhones when we updated them to iOS 4 or purchased an iPhone 4. Hackers have been teasing us that an iOS 4 jailbreak is imminent, but now you don’t have to wait for them or the jailbreak to have tethering again. You can have tethering on your iPhone now — using an open source project hosted on GitHub called iProxy.