The year before, over a $200 surcharge that the carrier levied on customers who had not fulfilled most or all of their two-year contract. Under pressure, AT&T relented, giving immediate access to the lower prices to customers eligible during a three-month stretch after the iPhone 3GS debut.

"It's safer to say that [trade-in] volume will increase if AT&T does what it did last year," said Trachsel. "It creates a bigger pool of people who may want to trade in their iPhone 4."

NextWorth and Gazelle wipe incoming iPhones, refurbish the devices, then sell them direct to consumers via eBay and Amazon.com, as well as to resale wholesalers.

Scarsella said there was a relatively narrow window for iPhone owners who want to trade in their older smartphones.

"The best time to trade in is from now until the day the new iPhone goes on sale," said Scarsella. "Although prices may stabilize or even rise then, once the new iPhone is out, prices will quickly drop."