I never had a problem at ATL when I was there surprisingly.
I had a 3 hour layover at ATL, so I went to the north end of concourse C and pulled out the telephoto and started taking pictures and no one said a thing. I also shot from the garage.
Now my home town airport MCO (Orlando) is pretty bad. I got busted on property a couple of times - once back in 2004 and I didn't have a camera, I was just watching planes - they just chased me out.
However this past February, a fellow spotter and I were on airport property taking pictures when a guy came out from one of the cargo buildings and warned us that if Ops caught us we could get in trouble. We thanked him for the warning, and decided we should leave after we took a picture of a "dime a dozen" Air Tran 717s landing. That was stupid because we probably could have got out fine. However 3 Ops vehicles suddenly surrounded us and took our I.D.s. Fortunately they were cool and professional about it, however they were also firm never to be caught on property again spotting. You can get special permission to spot at MCO they said, but they put you on the garage roof for that. You can get good pictures of planes at Airside 1 gates, but for any operations you would need at least a 400 mm telephoto. I was careful to get out of there because at the time I had applied for various operations positions at the airport and I didn't want to ruin my career. Fortunately that didn't stop them from hiring me because I start work with Landside operations soon at MCO.
Fortunately, my new apartment is in the approach for Runways 18L and 18R at MCO, and I live 3 miles north. My 300mm is sufficient enough to get good afternoon pictures in front of my apartment of the planes arriving, although 18R is practically a belly shot. I have parking lots down the street also I can get from as well as morning shots from a nearby park where no one bothers me.
DTW (Detroit) - This is a place where I have been bothered at times, and left alone at others. This was my old hometown airport.
When I was a kid, there was an unofficial spotting location called "Checkpoint 6" at DTW. It was right at the approach for what is now Runway 22L (then 21R). This was the best runway because it is 12,000 ft so all international stuff operated on it. It wasn't an official spotting location, but on weekends there would be 20-30 cars there with people watching planes and the cops would only come by to make sure no one was boozing it up or smoking joints (It got really popular after the Wayne's World movie - where Wayne and Garth were sitting at the airport watching planes land).
In 1996 it was closed and grassed over. People parked at the approach on the shoulder, but it wasn't long before the cops would come by and chase you away. I used to be able to park along the shoulder around the perimeter of the airport with no problems, but the cops around 1996 started chasing people away - possibly due to the then DTW direct to PEK route that was started, I believe then was the first U.S. route to mainland China on an American carrier. Also, this was shortly after TWA 800 expliosion, plus the fact that the Detroit area has the largest Muslim population in the U.S. got people all paranoid.
Since then its been here and there at DTW.
I got chased away off property near DTW by the police, but the week before I took pictures from the parking garage for 4 hours on prperty with no one bothering me.
Another time I was off property, some "Git Er Dun" type driving his pickup withthe giant American flag in the window and Bush/Cheney 2004 stickers still on it drives by me with a funny look on his face, makes a U turn and drives over to where a parked cop car is and the next thing I know the cop is questioning me. The cop was pretty cool, just checked my ID, saw I was no threat and told me I was ok there.