HTC has given the option to its users to unlock the boot loader on their phones if that is what they wished for. HTC has given this options last year but the AT&T One X was not part of the list of handsets for which HTC is offering the unlock. AT&T was the main reason of not giving the unlock option for One X but HTC never mentioned that officially.
Of course, it was only a matter of time before the people at xda-developers, who view restrictions as hurdles instead of a dead end, managed to get the phone unlocked and as usual they have come through.
For all the instructions and details, click here. You get all the necessary details and procedure to get this change done.