As you know HTC will be available in U.S. with AT&T, Sprint and T-Mobile, however, not all carriers-branded HTC One are ready for customization. According to rumors, the AT&T version won’t be supported by HTCDev, in other words, the bootloader is not unlockable, which means it will likely be difficult for us to obtain root or run custom ROMs.
The good news is coming to Sprint and T-Mobile users, most likely Sprint and T-Mo variants will be supported by HTCDev for unlocking the bootloader. You should know, in order to achieve root access have to inject root exploit into the device’s system partition, this can not be done when our bootloader still locked. To be able to unlock the bootloader, it should be supported by HTCDev.com to generate the unlock code for our device.
On this situation is estimated that the AT&T-branded HTC One won’t acquire root instantly, most likely the devs need more time to be able to obtain root access on this device, so don’t expect to be able to flash custom ROMs immediately on this device. However for flashaholic you are advised to purchase the developer version of this device, which comes with an unlockable bootloader, of course with the full price without a contract.