After MoDaCo's recent report that HTC's Bootloader Unlock tool didn't work for AT&T's One X variant, The Verge reached out to the Taiwanese manufacturer, and received a reply which suggested that the device has "restrictions" which prevent its bootloader from being unlockable:
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."