Another great news for the developers. The AT&T HTC One bootloader ca now be unlocked. This unlock is legal as it comes from he HTC's Dev website itself. The XDA users have been reporting about gaining root acces in HTC One and on flashing a custom recovery onto the AT&T HTC One. Though its a good news for those developers who wants to play on HTC One, the HTC have already mentioned that the bootloader unlock won't work for long as AT&T won't allow bootloader unlock for its variant of HTC One. So that says this window of opportunity is to be short lived.
Screenshots of the rooting & flashing on the AT&T HTC One indicate that it shares the same steps as those followed for the International version. Further updates from those who tried indicates that the flashlight, ringtone trimmer, sound recorder, and calculator applications are removed which seems fine. As always, once you unlock the bootloader of your device, root, and flash a custom recovery on to it at your own risk.