Some preliminary tests are showing that users can pop an AT&T or T-Mobile SIM card into a Verizon-branded iPhone 5, and you can get HSPA+ service as normal (assuming, of course, the GSM SIM is active). This is great news for T-Mobile customers that worried about having to unlock the SIM slot on their shiny new iPhone 5, as well as frequent flyers that regularly swap out SIM cards for local service. You can always shave down your own nano SIM cards, though it's generally not recommended.
Hopefully this will last, since the Sprint iPhone 4S launched with an unlocked SIM slot, but shortly after, they started locking it down again (though later offered unlocking to customers in good standing).
Anyone intend on swapping their SIM cards around a lot, or are you happy with your carrier? Did you end up signing a new contract when picking up your iPhone 5, or did you buy yours outright? If this is a limited-time deal, are you more tempted to get an iPhone 5 while the SIM slots are still unlocked?