I have never heard a law against it but I am sure that if you are snapping photos you may get questioned.

1) "In the United States, anything visible ("in plain view") from a public area can be legally photographed. This includes buildings and facilities, people, signage, notices and images. It is not uncommon for security personnel to use intimidation or other tactics to attempt to stop the photographer from photographing their facilities (trying to prevent, e.g., industrial espionage); however, there is no legal precedent to prevent the photographer so long as the image being photographed is in plain view from a public area."
"Some other restrictions on photography exist in the US, but most have to do with either commercial use of a space, such as forbidding photography inside a private building, or national security, such as restrictions on airport security areas or military installations"
Source and further information:
http://en.wikipedia.org/wiki/Street_photography


2) Here a discussion about this issue:
"Taking photos in airports forbidden?"
"In most airports it's prohibited to take photos in or near the security screening areas, but apart from that it should be ok. Also beware of photographing airport police or security personnel -- while you mightn't even notice that they are in the frame of your photo, they often take exception to being photographed."
http://www.flickr.com/.../


3) "Despite misconceptions to the contrary,
the following subjects can
almost always be photographed lawfully
from public places:
accident and fire scenes
children
celebrities
bridges and other infrastructure
residential and commercial buildings
industrial facilities and public utilities
transportation facilities (e.g., airports)
Superfund sites
criminal activities
law enforcement officers"
Source:
http://pdf.textfiles.com/pamphlets/thephotographersright.pdf


4) Here some ideas for your shots in airports:
"Shots That Take Off. Shooting Air Travel "
http://www.dreamstime.com/.../...travel_blog_art26


5) Outside the U.S., specific rules could apply!
Same for military airports:
"The agents asked him if he had taken any photos inside Camp Delta.
No, he replied.
The agents knew he was lying.

Al Halabi loved to take pictures. From the moment he had arrived at Guantánamo, he was snapping photos of his quarters, the beaches, his new friends on the base. He posted them on his personal Web site, along with messages to his bride-to-be.

To him, the pictures were a record of his American journey."
Source and further information:
http://seattletimes.nwsource.com/.../..._yeechapter5.html

I know in Cyprus you are not allowed to take photos outside of the airport ie. the runway where the planes are all sitting. I nearly got myself arrested for taking a pic of my son going up the steps of the plane, the hostess ran at me full tilt and grabbed the camera off me and almost ran up the stairs of the plane dragging me with her. She told me that because its also like an airforce base (??) i could be arrested for spying just by taking pics of a plane!! that was about 5 years ago, dont know if the rule still applies.

It is illegal to take pictures in security areas, otherwise it is legal (assuming your picture does not violate copyright or publicity rights, which will depend on what you are shooting).

It used to be illegal in the 1980's to take photographs in airports (at least in Canada) but that law seems to have disappeared.

Yeah...why not?