Well, according to some reviewers on the net, their 4G network, WiMax, has been rolled out in the Chicago area for a few months now. Chobbs1 is correct in saying that it is data only right now. They have no phones that work on the WiMax spectrum.
That being said, I am sure that Sprint will have some phones in testing to utilize the WiMax, but not release them until the network is rolled out in a majority of markets. They could be phones just like we have now, where they work on CDMA if there is no WiMax signal. If the signal is present, you use the WiMax as your network. It is not hard to imagine as T-Mobile utilizes this same type of system right now, albeit on a smaller scale with their WiFi calling.
Sprint could also be trying to split their bandwidth and use two separate radios, one for voice, one for data. Like 3G, so voice and data can run concurrently. Just a thought.