Another 12-18 months would be a perfect time to release a new iPhone that has been designed for GSM/LTE.
I'm not completely familiar w/ LTE--is it an evolutionary step to 1x/EVDO, meaning that an LTE device can fall back to those earlier modes?
If not, then it doesn't seem like a GSM/LTE hybrid would be a magic bullet for VZ as it would require them to switch on LTE networks in a massive number of territories within a very short amount of time--or else be seen as offering only a tiny footprint for iPhone 4G users to operate within.
GSM/LTE makes more sense for AT&T however as when 4G is not available the device could at the very worst fall back to 2G.