Mark is essentially correct. In a public space there can be no expectation of privacy, no permission required to snap away. Street photography sold as prints or in books is not considered commercial, it is considered editorial/artistic which is enough CYA for most street photography. It's when you sell the images for stock or advertising (or other "non" editiorial or artistic use) that model release is required if there's any one recognizable in the photograph.

Brian's link is spot on.

Of course IANAL, YMMV and all of this really only applies in the United States. Other countries may have different rules.