One topic that came up on the LCA2010 "chat" mailing list was a request that people exercise some consideration when taking photographs of others, particularly consider asking permission first. Out of the discussion that covered everything from the "Facebook" view (privacy is no longer a social norm) through to whether the venue counted as a "public place" (and if people who didn't want to be photographed should avoid "public places"), came some pointers to writing about law in New Zealand and Australia regarding photographer's (and model's and other content) rights.
I'm linking them here for future reference:
Clendons background paper on photography in New Zealand (pdf; cached copy), which includes a discussion around model's rights and model releases (as well as copyright, etc)
New South Wales, Australia, guide to Street Photographery Legal Issues
Aside from the law, as one person pointed out, taking photographs of someone against their will (rather than just without explicitly seeking their permission) seems ethically wrong in most situations even if it's not necessarily illegal in some of those situations. Often one can fairly easily obtain tacit consent (indicating a desire to take a photograph perhaps with a gesture, and being aware of someone showing they'd rather you didn't or expressing willingness); for a lot of casual photographic uses that may well be sufficient most of the time. (Commercial use being the most obvious one where not having anything written down, or even spoken, could later prove to be a problem.)
As always many issues of social interaction are better solved by social means, than by forcing them through a detailed legal analysis.