HTC has confirmed that they are not going to release the official unlock for the AT&T edition of One X.
The company said:
HTC is committed to listening to users and delivering customer satisfaction. Since announcing our commitment to unlockable bootloaders, HTC has worked to enable our customers to unlock the bootloader on more than 45 devices over the past six months. In some cases, however, restrictions prevent certain devices from participating in our bootloader unlocking program. Rest assured, HTC is committed to assisting developers in unlocking bootloaders for HTC devices and we’ll continue to unlock additional devices in the future.
It is pretty clear that AT&T is blocking them from releasing the unlock for this phone. Users would have to look for unofficial methods to unlock the device if they want to run custom ROMs on it.