It seems like some people here are right and some are wrong. My understanding is this
If you are in the middle of your contract and want to buy the phone at full price during pre-order you will be able to. It will say you're ineligible for an upgrade and allow you to buy the phone for the unsubisidized price.
However, if you do not have a contract and are just trying to preorder the iPhone for full-price then you will not be able to. This is how Apple and the carriers prevent price gouging and reselling.
If you are in the middle of your contract and want to buy the phone at full price during pre-order you will be able to. It will say you're ineligible for an upgrade and allow you to buy the phone for the unsubisidized price.
However, if you do not have a contract and are just trying to preorder the iPhone for full-price then you will not be able to. This is how Apple and the carriers prevent price gouging and reselling.