We don't have an official unlock through HTCDev yet, but that hasn't stopped the bootloader on the AT&T One X from being unlocked. After days of watching the devs do their thing on XDA, it seems that they have finally cracked it. This is the first thing we need to be able to flash all those custom ROM's we love, the next being a working recovery, which will probably follow suit in the next couple of days. Of course, there's a possibility that you'll brick your phone doing this, and it does involve a HEX editor and ADB skills. The thread has exploded, but it's worth doing the reading to see what kind of issues people are having, and what steps my give you some trouble. Hit that source link for the steps, and let us know how it went for you in the forums.
Source: XDA-Developers
