I've never been stopped or question in an airport, but I was detained in the Port Authority bus terminal in NYC once. The officers were not mean or bossy, and were responding to a call from a shopkeeper about a "suspicious man with a camera". I had setup a tripod and was framing a sculpture against the movement of the people in the background.

I explained to the officers what I was doing, showed them a few photos already composed, and then presented my ID. They were respectful, but still took me over to their precinct where I spoke to the desk sergeant. At the time, I worked in the NYC EMS and presented my badge and ID, along with some people in the PAPD who could vouch for me. With that, he let me loose and the officers accompanied me back to take the photo.

Still, in my discussion with the officers, I pointed out that far more technical and detailed photos were available from the Port Authority website than I was taking; and furthermore that the guy with the tripod in plain site was probably less of a risk than the guy with a point and shoot walking around.

In the end, I did get the shot I wanted; but I am not happy that they only let me do so because I was "on the job" and knew someone at the PAPD.

I've since checked out a few things, and came across "The Photographers Rights"; which I also now carry with me. I've never been questioned beyond that one instance, but if it happens again I will present this instead of my credentials.

The Photographers Rights:
http://www.krages.com/phoright.htm