Just wanted to throw in my two cents that my experience has borne out everything akuska has said. I had a friend who wanted to buy an iPhone 5 to take to Cyprus. At that time, you could buy the official unlocked GSM-only model -- which was the same hardware as the AT&T and T-Mobile models.
We went to the Fifth Avenue Apple Store, and of course they suggested she get the "unlocked" model, but I advised she get the Verizon model, since it had the LTE bands found in the European-only model. She paid full price and didn't have it activated or enrolled. She wasn't a Verizon customer. She just took the phone and used it.
It seems the confusion here has to do with the fact that the non-Sprint phones are in fact unlocked when they are sitting on the shelf at the retail Apple store, and are only locked during the process of selling them to you. If you don't need a contract or activation at the time of sale, they remain unlocked. Of course this isn't officially described anywhere, but those are the facts on the ground.
An associate actually told me this is how it works, when I was trying to understand it; she said, if I remember it correctly, that they only had two kinds of phones back there, the Verizon one and the unlocked GSM-only one. The "AT&T model" is what the unlocked one becomes during the sale process. (If you think about it, this makes sense, since it significantly simplifies inventory.) But before they released the "unlocked" model, was there any difference in what they stocked for the AT&T model? No. It was the same unlocked phone.
I think the confusion further comes from the fact that this doesn't at all apply to online sales, or carrier sales -- and that, on its face, if Apple isn't yet offering an official "unlocked" model, it implies that the models they are selling must be locked. It also seems unlikely that what you can get in the Apple retail store would be different than what you can get in their online store, but that's how it is. In the Apple online store, you can't buy a Verizon phone without a contract. But in the retail store, you can, if you ask.
So yes, it's entirely possible that someone could spend full price on a phone to get it on a month-to-month contract, but still have it be carrier-locked. But if they spend full price on it at an Apple retail store, and request that it not be activated (what akuska is calling a "device-only" sale), then that phone is unlocked, even if it's not the official "unlocked" model.
And finally, just to say it again, every new Verizon iPhone sold anywhere is GSM-unlocked -- just search "verizon iphone 5s unlocked" if you don't believe me, and you'll find reputable articles saying as much. The same should be true for the full-price T-Mobile phones sold in the Apple online store.
