Sprint has apparently already began building its own 4G LTE network, sounds like it is included the Network Vision upgrade that is already underway. Should be usable in early 2012. More to come Oct. 7.
Sprint to launch own 4G LTE network in early 2012 (scoop) | Wireless - CNET News
Sprint to launch own 4G LTE network in early 2012 (scoop) | Wireless - CNET News