Two of my friends just got iphones... pretty sure it would be like this... but I'd go in there and ask them... or call them and ask.
If your contract is up, you probably are eligible to renew it in full, and get one of their deals on the new phone as such, of course your contract would likely go up for the costs.
If your contract isn't, you could upgrade your service (and maybe have to extend contract for it) and have to buy an iphone at full price.