This is an interesting topic for me, because it is an issue that was raised in an ethics class I took at Ryerson when completing my journalism degree. Legally, taking someone's picture in public in Canada is subject to two different standards of law. In every province, except Quebec, it is assumed that when someone is in public that they have no right to privacy (with certain restrictions that I'll explain later). This was an issue for us as journalists, because, of course, with the cameras rolling (so to speak) sometimes we take pictures of or film people who do not want to be photographed or filmed. In the situation involving Quebec however, there is case law where someone has successfully sued a photographer for violating their privacy for taking their photograph in a public place without their permission.
There are some exceptions however. Stalking someone, following them or generally harrassing them by taking a picture could be construed as a criminal offence. Taking a picture surreptiously with a camera hidden so as to capture intimate parts of her body is also a criminal offence. Obtaining a picture under false pretences (ie claiming you're a talent agent) could also be construed as a criminal offence given the fact that she/he is consenting to something you're not qualified to offer.
Just as a general rule, I would suggest you get someone's permission before taking their photo. I know for myself that I would not only be suspicious of but would resent someone invading my personal space by taking an unsolicited photograph. Whatever you gain with the element of surprise as a photographic voyeur, you lose in the process of violating another person's space.
I think most people are sophisticated enough today to know that photographs can and do get manipulated all the time. I doubt anyone, unless they're young or naive, would knowingly allow a stranger to take a photograph.
