Sprint is cashing in on its investment less than a year after it contributed its entire 2.5 GHz spectrum holdings to Clearwire’s mobile WiMAX effort.

Yesterday, the carrier launched 4G service in Las Vegas, Portland and Atlanta over Clearwire’s WiMAX network. The carrier previously announced it would launch 4G service in 17 new markets this year. The new markets are concentrated in Texas, with deployments also planned in Charlotte, N.C.; Boise, Idaho; and Bellingham, Wash.

The carrier claims its 4G service is three to five times faster than the 3G service offered by any carrier today, based on average download speeds. According to Sprint, its mobile WiMAX service delivers peak downlink speeds of more than 10 Mbps and average downlink speeds of 3-6 Mbps.

Next year, Sprint expects to launch service in several market including Boston, Houston, New York, San Francisco and Washington, D.C. The company also plans to build out its device ecosystem with a single-mode 4G data card, embedded laptops and a 4G handset. It currently only offers a 3G/4G dongle.

Sprint is taking aim at the enterprise segment with unlimited 4G and 4G/3G data plans available exclusively to businesses. The unlimited corporate accounts come with several restrictions, including an off-network roaming threshold of 300MB per month and peer-to-peer file-sharing applications. The unlimited accounts are not available to consumers.

Clearwire is in the midst of a massive buildout for its 4G mobile WiMAX network. To date, the company has spent $646 million on the effort and says it is on track to reach its previously stated goal to cover 120 million people in 80 markets by 2010.

Clearwire investors Time Warner and Comcast have also begun reselling the service.