Sprint will offer all-you-can-eat data plans with the iPhone, helping to set itself apart from competitors AT&T and Verizon Wireless, which have phased out their unlimited data plans for the devices.

"Sprint's unlimited data plans provide more value than metered plans from other carriers," Sprint CEO Dan Hesse said. "Our customers can take advantage of the innovative features of the iPhone without worrying about overage charges or surprises on their bill."

The plans start at about $80, including Sprint's $10 extra data charge for its smartphone customers. Customers who want to use the device for a mobile hotspot have to pay an additional $30 per month.

Sprint began selling the iPhone 4 today. It's charging $100 for the 8 GB model of the older device, and customers have to sign a two-year contract.

Sprint customers can pre-order the new iPhone 4S beginning tomorrow or line up for its appearance at retail stores on Oct. 14.

The iPhone 4S costs about $200 for the 16 GB model, $300 for the 32 GB model and $400 for the 64 GB model.

The iPhone comes at a steep price for Sprint. The operator is rumored to have agreed to buy 30 million iPhone's from Apple over the next four years. With a postpaid base of 33 million subscribers, Sprint will be counting on upgrading its contract customers to the device. Like other operators, Sprint is selling the phone at a loss to its customers and then recouping the money through monthly service plans.

The data-intensive iPhone could also affect the profitability of Sprint's wireless network, since the operator's unlimited plans mean it won't be able to charge extra for bandwidth hogs. Hesse has told investors Sprint would stop offering the plans if they become unprofitable.