Found in today's (3-OCT-2010) Slashdot:
The Plane Finder AR application, developed by a British firm for the Apple iPhone and Google's Android, allows users to point their phone at the sky and see the position, height and speed of nearby aircraft. It also shows the airline, flight number, departure point, destination and even the likely course-the features which could be used to target an aircraft with a surface-to-air missile, or to direct another plane on to a collision course, the 'Daily Mail' reported. The program, sold for just 1.79 pounds in the online Apple store, has now been labelled an 'aid to terrorists' by security experts and the US Department of Homeland Security is also examining how to protect airliners. The new application works by intercepting the so-called Automatic Dependent Surveillance-Broadcasts (ADS-B) transmitted by most passenger aircraft to a new satellite tracking system that supplements or, in some countries, replaces radar.
More details here: