Wow. Alot of you guys here know NOTHING. Sprint is transitioning over to LTE FROM WiMax. They've already signed a 15 year deal with Lightsquared. And, ALL GSM PHONES ARE TECHNICALLY WORLD PHONES. GSM is EVERYWHERE. Just pop in a local SIM and go. While CDMA is not. The iPhone 4s is a WORLD PHONE because it has a GSM AND CDMA radio inside. Your iPhone 4s from Verizon or Sprint will work anywhere, provided you get a SIM for whatever country you're in. And most people dont travel the world where they need this anyway.
I doubt Apple would sign Sprint on as a carrier if they didnt plan on getting LTE. Whatever iteration of the next iPhone that comes out will have it for sure. It'll be LTE only, and work on all 3(applies to overseas as well, obviously). Even then, all 3 carriers LTE networks operate on different frequencies, so it'll all be seperate as usual anyways.