1988, I'm visiting the Cambridge University campus when a guard starts yelling at me. I had set up my AE-1 with 28mm lens on a cheap $30 tripod so that I can make a record of my visit. He takes me to the administrative office, and explains that before I took any more pictures, I had to register my personal information, and sign a statement that the photos would not be for commercial use. Apparently, the tripod was the tip-off that I must somehow be a "pro".
A few years ago, I'm taking photos of my children during a concert at the local music conservatory. As usual, no flash, and I have my 5D and 70-200 f/2.8IS. Notwithstanding the warning at the start of the concert that flash photography would not be permitted, a parent sitting nearby snaps off a flash photo with her p&s. At intermission, a guard approaches, and tells me I shouldn't be taking photos. This year, the music director asked me to be the official photog for the concert. Before the concert, I approached the head of security, and graciously asked her for help in letting her staff know that I would be walking about taking photos for the school. They went out of their way to be accommodating. It pays to be nice.
Today, I'm at a public dog park with my two Samoyeds. I'm shooting images of my dogs in action with my 40D and 100-400IS lens. A half-hour into my shooting, a woman approaches me and asks whether I have obtained permission to take pictures of people. My response: "I am not taking images of people, but images of my two dogs at play. The people are only getting in the way, which is why I use this big lens to crop them out." She left with an embarrassed smile.
I'm more and more convinced that I need to get the 70-300IS DO lens for my future travels, particularly overseas. Either that, or my DSLR kit stays home in favor of my G7 p&s.
Somewhat off topic, but I recently came across this humorous and heartening video on Youtube of a guy who traveled the globe taking videotape clips along the way of himself and the locals dancing (there's even one of him dancing in front of a border guard on the Korean DMV zone). It reminds me that not everyone in this world is camera-shy or obsessively uptight about having their images taken: http://www.wherethehellismatt.com/videos.shtml