The nexus, the evo lte and the viper only support LTE on the G-block of the 1900 band where sprint is rolling out their initial LTE in a 5x5 spectrum. They have pretty much nationwide coverage of this band so most areas should see good coverage... as long as they build/upgrade a tower for the location. Building penetration should be similar to what we see today for 1x/3G.
Along with the LTE rollout they will also be rolling out CDMA 1x-advanced in a small section of 800mhz spectrum (from what I've been able to tell they didn't have adequate spectrum to deploy LTE here now since LTE requires 10mhz of bandwidth, minimum). This should help improve voice coverage in some areas where sprint has poor 1x/3G coverage today and provide basic service in rural areas where the 800 mhz spectrum gives them more coverage per tower. This 1x advanced CDMA network is the one that provides support for HD voice. The evo supports 1x advanced and HD voice... I'm doubtful that the nexus or viper do but I haven't heard anything one way or another for sure.
That's the stuff they are currently rolling out.
Then, in a year or two, they will expand LTE into the SMR spectrum in the 800mhz range (this network will co-exist with the 1900mhz LTE, not replace it). Sprint is in the process of transitioning the iDen push to talk customers to CDMA which will free up the SMR band for LTE. They will be taking advantage of advancements in LTE that allow them to bond the SMR LTE with the G-Block LTE to provide increased bandwidth. None of the phones announced support this yet because the needed technology isn't available.
This is my understanding of where things are today and where they are headed...