Since last year, HTC started giving users the option to unlock the bootloader on their phones if that is what they wished for. However, the AT&T One X was not part of the list of handsets for which HTC is offering the unlock. The reason for this was widely believed to be AT&T, although HTC never actually officially mentioned that.
Of course, it was only a matter of time before the people at xda-developers, who view restrictions as hurdles instead of a dead ends, managed to get the phone unlocked and as usual they have come through.
You will find the full instructions in the source link below, along with the usual dose of warnings and disclaimers. You will need a rooted phone, along with some experience with HEX editing and ADB and as long as you know what you’re doing, you should be fine.