Unlike most other countries where iPhone is available on multiple carriers, you won't be able to go to an Apple store and pay full price for an unlocked iPhone you can use on any network -- or any carrier around the world with a simple SIM swap. Verizon, being a CDMA rather than GSM carrier doesn't use SIMs.
Instead US customers will have the choice between two locked iPhones -- locked to AT&T or locked to Verizon, unable to move between the two, and not even able to officially move to the other GSM and CDMA carriers in the US, T-Mobile and Sprint. While we don't know for certain, in all likelihood you won't be able to call your carrier and get your iPhone unlocked, you won't even be able to pay them to unlock it.
That means if you're currently an AT&T iPhone user and you want to move to Verizon you have to buy a new Verizon iPhone. You can't take it with you. And if you get a Verizon iPhone and decide life on AT&T really was better, there's no bringing your new phone back.
Will the ever be a time when an American can walk into an Apple Store and buy an unlocked iPhone, capable of working on any carrier in the US, just like in most of the rest of the world? Because come this time next month the iPhone in the US -- land of the free -- will still be locked. It'll just be locked to two carriers instead of one.