I actually contacted the seller of those listings, this was our conversation:
Hi. I was just wondering, what do you mean when you say "Unlockable" in the title of your AT&T Nokia Lumia 920 listing? Could you share with me how to unlock it, or was that just a mistake when you put it in the title?
No it was not a mistake.. my friend has his unlocked for sprint. it was done buy one of those "we fix cell phone" places.. Thank you
Hmm, that's strange. Not sure how your friend could have got it working on Sprint; I don't think they use SIM cards. Do you know exactly which place, or no? If not, that's fine, thanks for the info!
So the seller probably just put that in the title to attract attention to his listings... if anyone finds out of any online unlocking services that actually can unlock the 920, or of any third party retailers that are selling unlocked 920s at a decent price please keep us posted.