While AT&T added more customers last quarter than competitor Verizon — thanks in large part to the iPhone — AT&T’s CEO admitted this week that his company won’t have exclusivity on Apple’s phone forever.

The concession by Randall Stephenson at a Fortune conference, as reported by PC World, comes as the chief executive is reportedly working behind the scenes to extend AT&T’s exclusive contract with Apple.

Stephenson’s comments were cryptic, at best. The CEO simply said that iPhone exclusivity with AT&T will not last forever. However, he did not elaborate. He didn’t need to, though, basically admitting (probably in 2010) they will lose their fotthold on one of the world’s most popular smartphones at some point. And it will hurt their bottom line quite a bit.

On Friday, Verizon Wireless announced that it added 1.1 million subscribers during the last financial quarter. But during the same frame, AT&T increased its total number of customers by 1.4 million.

Many of AT&T’s new customers are coming due to the exclusive iPhone contract. This week AT&T announced that more than 2.4 million iPhones were activated during the last quarter, with more than a third of those new customers.

That means of AT&T’s 1.4 million new customers in the June quarter, at least 800,000 were iPhone subscribers. So if they lose the iPhone, it could take a huge chunk from their new customer gains.