Bootloader for AT&T Branded HTC One is unlocked
The HTC Dev site will unlock your bootloader and the process to do it is basically the same as it is for the international version of the HTC One.
Thankfully, for those that like to dig deep into their gadgets, you will not have to look too hard to get into your phone (unlike the work that has gone into some Motorola products). A couple of side-effects of getting into the bootloader is that all the data on your device will be erased, and a couple included apps are removed as well, like Calculator, Flashlight, Sound Recorder and Ringtone Trimmer.
The only tangible consequence of that is that the HTC One will no longer be able to receive OTA updates. However, for most that like to do this kind of tinkering with their devices, this will not pose much of an issue.
sources: XDA Developers via Android Central