If I were an e-cig smoker on a flight, I think I'd just discreetly pop into the loo for a quick fix. The fine for doing that with a real cig would definitely be extremely high. But with an e-cig, no one but you be the wiser.
I am seeing more and more people using them quite openly in bars and restaurants now. My sister uses one and, when one customer complained, management explained it's perfectly safe and virtually odorless. And not under ban.