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.
I can't speak for Verizon, but if the iPhone 4S with the GSM SIM works similar to the other Sprint world phones (i.e. most new Blackberries, HTC Touch Pro 2, Moto Photon or whatever), then Sprint can activate the SIM for international travel. Depending on the country, data may or may not be available and an additional plan may be required or pay as you go.