The Truth about these locked phones
Buying directly from Appleor an Apple retail store (NOT Best Buy or anywhere else) is the ONLY way to get an unlocked iphone, and even then, it will only work with at&t or T-Mobile (GSM networks only).
If you buy one from anywhere else, regardless of what you are told, you will be getting a phone that is locked to a single network, and there's nothing you can do to fix it. Verizon may work with you under certain terms, but that's if you're lucky.
When they say "unlocked," they mean "partially unlocked" so that you can use non-US (i.e., foreign) SIM cards in the phone if you travel. When you're in the US, you are locked to the network of the mobile service provider that sold you the phone.
Go to the apple website to buy one contract free, and you will see this information provided at the bottom of the page.