http://www.usatoday.com/tech/columni...y-rights_x.htm

The law in the United States of America is pretty simple. You are
allowed to photograph anything with the following exceptions:

* Certain military installations or operations.

* People who have a reasonable expectation of privacy. That is, people
who are some place that's not easily visible to the general public,
e.g., if you shoot through someone's window with a telephoto lens.