Laws differ from each country but generally speaking, you need a model release if you are going to sell the image which involves
advertising a product. You can sell images for editorial use without the need for a release. There is an exception to this (of course). The image itself must not have a contextual implication - ie. if you sell an image of, say, a young woman, and you sell the image which will appear in an article on prostitution, you will definitely need a model release (and would be lucky to get one!) because there is a contextual implication that the person in the image is a prostitute. This is an extreme example but you get the picture - 'scuse the pun! Here is an interesting read for those in Australia (and it would possibly be more or less the same in the UK) -
http://www.4020.net/unposed/photorights.shtml
And for the US -
http://www.krages.com/phoright.htm
As I said, laws differ and I would check the laws which apply to your country. I know in Italy and France (or so I have been told), its considered an invasion of privacy to take any snap of anyone even in the public domain and you risk prosecution.
I take a lot of street shots and I now ask for model releases (after the event) and generally, folk don't have a problem with it. Its worth having a release where you share the fee you get for the image with the "subject" so it makes it worth their while.
Cheers
Sheila