From:
http://www.answers.com/topic/street-photography
Legalities
Your rights as a Street Photographer
General
Photographing without permission
In the United States, anything visible ("in plain view") from a public area can be legally photographed. This includes buildings and facilities, people, signage, notices, and images. It is not uncommon for security personnel to use intimidation or other tactics to attempt to stop the photographer from photographing their facilities (trying to prevent, e.g., industrial espionage), however there is no legal precedent to prevent the photographer so long as the image being photographed is in plain view of a public area.
On public property
See above.
Publication
In general, you can not publish someone's image to endorse a product or service without first acquiring a "model release," which is (usually) a contract between the publisher or photographer and the subject.
Defamation
It is somewhat difficult to imagine a hypothetical scenario in which a photograph, by itself, would be defamatory, since the key element of defamation is falsity. Perhaps if a person was photographed in such a way that made them appear to be engaging in some indecent activity, it could qualify as defamatory. Digital editing of photographs certainly opens the floodgates for defamation because it is easy to turn a formerly "true" photograph into one that does not depict anything near the truth.
Invasion of Privacy
In 1890, Samuel Warren and future Supreme Court Justice Louis Brandeis published "The Right to Privacy," which made their case for recognition of invasion of privacy as a legal tort.
Fifteen years later, in the case Pavesich v. New England Life Insurance Company, a Georgia court was the first to rule on the balance between the right to privacy over freedom of the press, when it found that Mr Pavesich had been wronged by the appearance of an unauthorized advertisement in which his photograph appeared. The court at that time ruled that commercial usage did not have the same press protections as other forms of use.
Earlier, in 1893, the case Corliss v. Walker had set the related precedent that non-commercial use, in this case an unauthorized biography, was indeed an example where press freedom's inherent public interest could not be overruled by the right to privacy. These two cases along with the abovementioned "The Right to Privacy" have become the basis for almost all US law with respect to the balance between freedom of expression and individual privacy.
Some other restrictions of photography exist in the US, but most have to do with either commercial use of a space (such as forbidding photography inside a private building) or national security (such as restrictions on airport security areas or military installations).
Also have a look at
http://www.photopermit.org/
Cheers,
Keith