VoLTE, is Voice over Long Term Evolution, meaning the use of one LTE radio to perform Voice and Data simultaneously. I believe this is performed used a Voice Over IP (VOIP) type of data connection (Like Vonage does on a home network), since LTE currently is used for data only.
Your current 4G-LTE phones use your 4G radio for data and the 3G radio is used for voice, giving the appearance of Voice/Data simultaneously, this can only happen in a 4G area. The HTC Thunderbolt uses a 3G SVDO radio that can do Voice/Data over a 3G network, but uses 4G Data and 3G Voice when on a 4G network like the others. The LG Revo has capability to do VoLTE, when it comes available, 4G Voice/Data-1 Radio. What this means to you...
Nothing really for the next two years, Verizon is not going to kill off the 3G network anytime soon, and it will be in more places than the 4G network for a while, but the good news is the 700MHZ/LTE spectrum is the old analog TV broadcast, Remember the federal government's hurry to switch over to TV Digital Broadcast? Well that was to facilitate the Cell Industries plan to use the lower frequency to penetrate buildings a little better.
IMO, the LG Revo is a little future proof with VoLTE, but seeing the landscape of how quickly technology changes and the way consumers rip through smart phones, I think it is a none issue. VoLTE is suppose to be a clearer signal, so that would be an advantage. Also the advantage will be evident in the handoff between cell towers, VZW said there will not be a handoff between 4G to 3G tower (Example, leaving LA going to 3G Area), so in most cases this will probably result in a dropped 4G call.