Verizon will not activate a phone unless it was sold as a Verizon phone. It isn't a technology thing it is a corporate policy. So if you want to use Verizon you need a Verizon phone. It can be off contract. You can go into Verizon and buy a full price phone and be off contract. And they will even give you a small discount on your bill. But you can't just use any unlocked phone.
But this is just a policy thing. So Verizon could change it at any time or make an exception for all unlocked phones bought through Apple Store.
That has also been my experience, but the ad copy on the iPhone website says otherwise. Still waiting for confirmation, one way or the other.