I can attest to the fact that once you unlock the bootloader, it won't get relocked since it's a physical change (blown e-fuse as others have mentioned).
I have flashed back to a previous stock, took the OTA and the bootloader was still unlocked.
However, never say never and when KitKat comes out for this phone, I will not be quick to install it. I will wait and see what everyone says before I accept it. No way I take an OTA that somehow finds a different way to lock the bootloader.
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RAZR HD XT926 (unlocked)
32gb Nexus 7