While each contract you sign is supposed to allow you to purchase a new phone at the subsidized price every 18-24 months, when a hot new model is released, it is up to the discretion of the carrier as to whether or not they will allow you to upgrade early. If you are currently on Verizon or AT&T, you can click on the sourcelink to see if you qualify for the lower pricing. And no, we have not forgotten Sprint. The nation's third largest carrier joins the Apple iPhone club with this launch, but the upgrade information for the carrier will be added later, according to Apple.
source: Apple via AppleInsider
Apple's web site will let AT&T and Verizon users know if they are eligible for an upgrade; Sprint information will be added later