An E-cig makes a hell of a lot of sense for use in combat. Many military folks smoke (damn near everyone when I was in though that was a few years back) But the cherry on a cigarette makes a very good target for a sniper (just aim for the bright red light .. boom headshot!!)
Something like a Prodigy or another manual smoke with no LED would work very well in that enviroment. The only problem I'd see would be getting power to it. Traditional smokes work well in the field since all you really need is a source of fire to use them and some sort of lighter has other uses than lighting smokes.
An E-Cig on the other hand requires a lot more supporting hardware to operate. Spare batteries, chargers, juice, atomizers, switches etc.
Still if one could be designed to run off some other equipment that soldiers tended to have with them anyway or posibly a solar powered recharger of some sort that might work.
Hell would be cool to build one that looks like a 20mm round would have room for a battery in one of those and would be funny to see someone puffing on a bullet.