[Alan]

This is a great website written by Bert P. Krages, Attorney at law. This article explains what your rights are as a photographer.

A Downloadable Flyer Explaining Your Rights When Stopped or Confronted for Photography

http://www.krages.com/phoright.htm

[equetefue]

this is a good idea. I always carry one like these in my bag. You just never know.

Still best solution for a bad encounter is common sense. It might not be worth the arguement

[mullenite]

It usually isn't worth the argument, it's usually actually a pretty good idea to ask first if you can. If you can't just don't be in anyone's way and don't cause a scene and you'll usually be fine.