Greetings,
This information applies to the USA only (including Hawaii and Alaska). If you are in Canada or Mexico or another part of North America outside the USA you will want to consult their laws.
In the post September 11, 2001 world people with cameras are under constant scrutiny. Many of us have experienced harrasment, detainment, physical confrontations and threats. Some of it very minor... and some more severe.
Go here, read the article and print out many copies of the PDF to hand out to people, police and rent-a-cops that question/harass you. Know your rights and make sure they know your rights too.
MTFBWY,
--Will