It's part of Apple's "Reuse and Recycle Program." When you go to the company's website, you can select the model, color and condition of your iPhone 4S. Apple then determines what it's worth. If you agree to the price and are able to send the power cable along with it, Apple will send you a gift certificate that can be used towards the purchase of a new phone.
Reports suggest the new iPhone will be announced September 12th.