This version is SIM-Unlocked for carrier interoperability, but not BootLoader-Unlocked. That version will be $650 and has been delayed.
Unlocking the bootloader for unlocked HTC One's is a relatively simple process (and well documented), requiring a visit to HTC's dev site.
AT&T will force HTC to disable the ability to unlock the bootloader for their One's sold subsidized, but that isn't the case just yet.
I don't see HTC One as one of the supported models on the dev site.
Can you use the same process as one of the listed devices?