I was told that certain plane spotting activities like taking photos of aircraft taking off and landing could be illegal in the US since 9/11? What are the regulations on this, and what should I be aware of before I set my camera up at the end of the runway?
Generally absent a specific prohibition you can take a photo of anything you can see with your eyes from public property (like a sidewalk). That includes airports, airplanes, bridges, tunnels, police officers, kittens, and unicorns.
(For more in-depth information check out Andrew Kantor's excellent article and Bert Krages' excellent one-page flyer on photographers' rights, or check out http://photography.stackexchange.com - they've covered airport photography, though not in any real depth, and I'm sure they have some street photographers well versed in the legal aspects of taking pictures.)
So what constitutes a "specific prohibition" on photography? Generally a clearly visible sign that says
PHOTOGRAPHY PROHIBITED (the old Grumman facility in Bethpage had such signs on all of the fences where there was a decent view of the runway).
Absent such a clear prohibition you're free to take all the photos you want as long as you're on public property while you're doing it (i.e. "not trespassing on airport property"), but be aware that some law enforcement officials may not see it that way, and you may have to have some uncomfortable interactions with them.
In my experience most police officers are well versed in the law, though occasionally you'll find one who isn't. You're more likely to have trouble with a "rent-a-cop" security guard though.
If you want really good photographs (and you're not photographing at a major airport like JFK or O'Hare) you might want to speak to the airport management. At smaller airports someone - either airport personnel or a local pilot - might be able and willing to escort you airside where you can get some really great shots.
It helps in this situation if you have a body of work you can show (either published photos or art prints). The caveat is that many airports will also ask that you carry some kind of insurance while you're photographing on their property (and at larger airline fields that requirement can be substantial).
The ACLU notes that:
The Transportation Security Administration (TSA) acknowledges that photography is permitted in and around airline security checkpoints as long as you're not interfering with the screening process. The TSA does ask that its security monitors not be photographed, though it is not clear whether they have any legal basis for such a restriction when the monitors are plainly viewable by the traveling public.
The TSA also warns that local or airport regulations may impose restrictions that the TSA does not. It is difficult to determine if any localities or airport authorities actually have such rules. If you are told you cannot take photographs in an airport you should ask what the legal authority for that rule is.
Popular among photographers, The Photographer's Right, a summary of legal rights related to photography (prepared by a lawyer), says that:
There are some exceptions to the general rule. A significant one is that commanders of military installations can prohibit photographs of specific areas when they deem it necessary to protect national security.
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), criminal activities, law enforcement officers
Some airports are more spotter-friendly than others. You can search online to find discussion forums on people's opinions which ones are better than others. Any airport that is private property (not many major ones, I believe) of course can do whatever they want, but many airports stop photography for a "safety" measure.
Standard IANAL disclosure.