U.S. cell phone service provider Sprint is accepting pre-orders for the iPhone 4, and says it will begin taking pre-orders for the iPhone 4S on Friday.

The addition of the iPhone to Sprint’s arsenal is certainly good news for the beleaguered carrier, which has lost customers to its rivals despite a steady television ad campaign and the promotion of unlimited calling and data services. Sprint CEO Dan Hesse told attendees at a Goldman Sachs conference in September that the iPhone’s absence was the number one reason why Sprint lost customers.

Sprint is differentiating its entry into the iPhone market by offering customers an unlimited data plan. Neither AT&T nor Verizon offer such plans to new customers anymore. Hesse said that unlimited service is profitable for Sprint, but made that comment before the iPhone – a notoriously bandwidth-intensive device – became available on their network.

Hesse said that if Sprint’s unlimited service plan proved to be unprofitable, they won’t offer it anymore. Given AT&T and Verizon’s lead, it would seem to be more a question of when than if.

Apple announced Sprint as a new iPhone carrier during the introduction of the iPhone 4S on Tuesday. Sprint is now the third major carrier in the U.S. to offer the iPhone 4 (and 4S) to customers, leaving T-Mobile as the sole major carrier which doesn’t offer an iPhone.