Preview at WWDC likely in June, followed by September launch.
Apple Confirms iPhone Trade-In Program Launching Today at U.S. Retail Stores [Updated]
Under the program, users can receive a gift card in exchange for their older devices, although the gift card must immediately be used toward the purchase of a new iPhone. Trade-in values are based on a number of criteria, including model and capacity and physical condition.
The trade-in program launches less than two weeks before Apple is expected to introduce its next-generation iPhones at a media event on September 10. Rumors and speculation indicate that the new iPhones could be available in the U.S. and other first-wave markets beginning on Friday, September 20.
Update: More details are coming out about the trade-in program. Apple is calling it a "reuse and recycling" program and trade-ins for working iPhone 4 and 4S devices ranging from $120 to $250, according to TechCrunch. Customers must use the money to purchase a new on-contract phone.
Apple provided this statement to TechCrunch regarding the new program:
iPhones hold great value. So, Apple Retail Stores are launching a new program to assist customers who wish to bring in their previous-generation iPhone for reuse or recycling. In addition to helping support the environment, customers will be able to receive a credit for their returned phone that they can use toward the purchase of a new iPhone.