HTC One X

I plan on picking up a SIM-unlocked GSM phone for use with T-Mobile. Now, the bootloader is not going to be unlockable on the One X, but I saw someone say that if you don't get the AT&T version, it will be unlockable. My question is: what exactly constitutes a One X that isn't that AT&T version? I mean, if I were to buy a SIM-unlocked One X, couldn't it have been an AT&T version that someone unlocked the SIM on, and thus I wouldn't be able to unlock the bootloader? Is there a SIM-unlocked One X that I can use in the US and also unlock the bootloader?

I hope my question makes sense, let me know if I need to clarify.