The article suggests that buying a Verizon iPhone will give you LTE service on AT&T. This is not the case, as the two phones support mutually exclusive LTE bands. The AT&T iPhone 5 supports LTE bands 4 and 17, while the Verizon iPhone 5 supports LTE bands 1, 3, 5, 13 and 25.
I don't know whether Verizon has confirmed this will not happen with the iPhone5, but as one report I read described, they have in the past shipped unlocked handsets that were subsequently locked down by an update. (Sorry, I don't recall the details and don't have the URL at hand.)<p>So... I'd confirm that the unlocked GSM setting will persist, before committing my money on the basis of its being present.