From “Homeland Security Warning: Could a Watch become a Weapon?,” from WABC, :
According to U.S. intelligence sources, Al Qaeda is trying to obtain watches with a hidden butane lighter and watches with an altimeter built in, that together would be used to bring down commercial airliners.
Pan Am Flight 103 was brought down over Lockerbie, Scotland with a bomb that used an altimeter device to set it off. Al Qaeda, according to intelligence sources, is adapting that technology and trying to combine it with the butane releasing watches that are designed to be used as cigarette lighters.
Altimeter watches show how many feet above sea level a plane is. They are inexpensive, though they are not considered terribly accurate. In Pan Am Flight 103, the bomb was set to go off when the altimeter indicated the plane had reached a certain height.
The U.S. Department of Homeland Security bulletin specifically mentions Casio altimeter watches as the most readily available, although there are other brands out there. Watches that can be used as lighters are available on the Internet, and are also inexpensive.
This would make two crucial bomb making components easy for Al Qaeda to smuggle onto an airplane.
Watches are expected to make metal detectors go off, they are not given much scrutiny at airports. Richard Reid, who once smuggled a bomb onto a plane in his shoes, showed that the rest is frighteningly easy to get on board as well.