Also note that some places, such as some provinces in Canada (Manitoba and Quebec for sure, there may be others) have "privacy laws" that make it illegal to use someone's indentifiable image for profit, regardless if that image was taken on public property.
For instance, where I am in Manitoba, the Privacy Act specifically prohibits using someone's likeness "for gain" without consent. To my knowledge this hasn't been tested in court here yet (although the Quebec law was tested in Quebec a few years ago with a woman who was shot as part of a crowd, that shot was hung in a gallery, and the woman didn't appreciate it and won in court). I know a few street photographers here who basically just ignore the law for now.
So, as far as I see it, it is legal to take someone's picture on public property in most places, but it may not be legal to do anything with that picture once you've taken it, depending on the law where you live (and/or where the pic was taken), and if anyone is "identifiable".