There are a few existing discussions on e-cigs on PPRuNe, such as here:
http://www.pprune.org/passengers-slf...06-e-cigs.html
I know Qantas do not allow them - not sure about other carriers.
That said, it's totally legal to carry them on board an aircraft, ideally dismantled, and I'm sure pax, particularly on long-haul, 'smoke' them discreetly in their seats, or in the lavatories.