Tether’s new version for the iPhone is purely based on HTML5 and creates a completely wireless connection over AdHoc. This circumvents the need of buying the application directly from Apple’s App Store and allows any iPhone or iPad with a data connection to allow tethering.
No need for an iPhone app anymore.
The question comes, though. Can AT&T detect it? If they can you just got upgraded. (And lost the grandfathered pseudo-unlimited plan).