Scarsella called Gazelle's trade-in volume "unprecedented," and said that showed intense interest in the iPhone 5. He attributed that increased interest to the longer period of coverage by news media and blogs because Apple departed from its usual June or July launch with the iPhone 5.

"This model didn't follow the typical Apple release, and the later [launch] created even more hype about the iPhone 5 or iPhone 4S, or whatever they call it," said Scarsella.

Gazelle has also seen a spike in trade-ins of Android and BlackBerry smartphones in the last three weeks -- a 102% increase in the former -- which Scarsella interpreted as a rush by those consumers to dump their phones in preparation for buying a new iPhone.

The number of U.S. iPhone 4 owners who decide to sell their existing smartphone will get an even bigger boost if AT&T decides to relax its rules on who can upgrade to a new model at the subsidized price, said Trachsel of NextWorth. "But it remains to be seen if carriers will let customers trade up by locking them into another contract," he said.

Last year, AT&T announced that any iPhone owner scheduled for upgrade pricing between the launch of the iPhone 4 and the end of 2010 was immediately eligible for the lower, subsidized prices of $199 and $299.