The Presidential CAP (Combat Air Patrol) is a standard security measure in the post-911 world. There is normally a CAP over the President whenever he is on the ground. Since Presidential movements are often not publicized very long in advance, it is easy for a Sunday flier to unknowingly wander into the protected area. Many small planes have only one receiver and often do not monitor guard frequency. The CAP aircraft, usually F-16's, try to identify any intruders but often have a hard time flying slow enough to get the N-number on light planes. An AWACS and tankers are usually involved, AF1 had CAP coverage over the Atlantic on the recent crossing which is unusual in my experience.
Many countries try to provide a wingtip fighter escort for AF-1 as an airborne honor guard. The gesture is appreciated but strongly discouraged in recent years.
You can hear a recording of a Presidential CAP over Daytona Beach, Florida several months ago here:
http://www.milaircomms.com/audio/032603-potus-cap.mp3
Here's a couple of articles on the city CAP's, very similar to the Presidential CAP missions:
http://www.aviationnow.com/content/p...2/avi_stor.htm
http://edition.cnn.com/2003/US/07/03...eserve.ahlers/