The most important piece of advice here is to get in touch with your airline if you wish to find out more about using E-Cigarettes mid-flight. Different airlines (domestic and international) usually have their own policies that need to be adhered to, but the simplest fact is that these are nowhere near as intrusive as lighting a regular cigarette would be. Users would argue that in terms of electronic interference, they are just as harmless as switching on an iPod, but it is never safe to assume that you can just pull one out and use it without checking first.
E-Cigarettes that are available in Canada will usually emit a small amount of harmless vapour, but at the same time this by-product may make other passengers feel a little uneasy. In the event that the cabin crew are happy for you to start using one, it would still be a good idea to ask those around you if they mind - they may even be naturally curious about your trendy flight accessory. If it is more considerate, you could always move to one of the galleys around the plane or a similar area.
You should bear in mind that in the grand scheme of things, E-Cigarettes are still a relatively new invention, and this means that many airlines will not have the full set of facts about them. If you are that passionate about them, you could even get in touch and explain why you would like to be able to use them in the air!