Maxwell: I didn't mean to give you the wrong idea. While trespassers could be shot, it probably would never happen and I don't think it ever has. There's certainly not a policy to shoot first and ask questions later. I know of a few facilities that failed their security tests a few months ago when people got on the grounds, into the buildings, and spent time walking through with a package. At one center the person (working for the government to test security) walked into the manager's office, set the package down and said, "Boom - that was a bomb and it just went off." Obviously, they couldn't get anyone foolish enough to do these types of tests if the policy was for guards to immediately shoot any and all trespassers. Theoretically the guards can shoot - it's expected they would try to apprehend the person first though. Likewise, I don't think a trespasser could really cause a mid-air unless that was their intention (even then it would be difficult) - and if that was their intention the sign wouldn't really be of much use.

Added: I just took a look at the signs you're talking about that have been there forever ... they do say something like ...warning interfering with an air traffic controller is a federal offense and could lead to loss of life (so yeah, talking about airplanes there, but pretty far fetched).