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.)
So... I'd confirm that the unlocked GSM setting will persist, before committing my money on the basis of its being present.
