I have followed this thread since its origin, and out of respect for basic human life, freedom, and dignity, I can no longer remain silent. The explanations about brotherhood, cycling fellowship, etc., are naive at best, and are deliberate attempts to cover up the truth at worst. I shall use the relative anonymity of Usenet to tell the real truth, for were I to reveal these facts through any forum which might identify me I would be at great risk of personal harm.
While it is true that many waves are harmless greetings or symbols of fellowship, there exists a vast group of motorcyclists which belong to a secret society. This Society, which has existed for centuries (and by some accounts, perhaps millenia), controls the world banks, a number of governments, most organized religions, a number of large corporations, a major restaraunt chain, and a significant portion of the print and television media. The goal of this Society is nothing short of world domination. These attributes have sometimes been associated with a "World Jewish Consipiracy", or other such nonsense. Such rumors, while not likely to have been started by the Society, are not discouraged as they divert attention away from Society activity.
Motorcyclists form the militia of this Society, and are responsible for conveying messages, orders, narcotics, and other small materials, in addition to being responsible for numerous acts of terrorism and sabotage. While organized motorcycle `clubs' such as Hell's Angels and the like may appear to be part of this conspiracy, in reality they are mere puppets of the Society, and have no direct function, but merely serve to divert attention from the true members of the Society. Most of the true members -- which easily number in the millions worldwide -- prefer inconspicuous UJM's, and typically hold down regular jobs, dress neatly, cut their hair, and outwardly appear to behave in a civilized manner.
The seemingly innocuous wave between the cycling members of the Society is actually a secret code which is used to convey messages or warnings. The system of coding is extremely complex and rich, and is exceedingly difficult for any but specially trained observers (or Society members themselves, who have been trained through advanced technology) to decipher, or even distinguish at all. It employs an intricate system of subtle and rapid finger movements to convey information, but to the casual observer this is rarely apparent. One significant identifying characteristic, however, is that the middle finger is always slightly upraised or otherwise prominent throughout the wave. This is often done by ensuring that the middle finger is somewhat longer than the surrounding fingers. It is thought by some historians that this may in fact be the origin for the popular "fuck off" gesture.
The most insidious aspect of the Society is that many of the "front line" members are members without even knowing it. Through advanced brainwashing techniques, they are rarely aware of the great harm they are causing, and only become aware when they are indoctrinated into the next level of the Society heirarchy, at which point, of course, it is too late. Members are almost invariably recruited without their awareness, however anyone fearing this evil conscription would be wise to watch out for door-to-door salespeople, television evangelists, or youth groups selling cookies (especially those containing chocolate, or Brazil nuts.)
I have been a third-tier member of the Society for most of my adult life, but due to recent events in Europe and Africa which suggest the leadership of the Society is becoming increasingly corrupt and destructive, I can no longer condone or participate in the behaviour of the Society. Since I have taken great care to shield my true identity, I can only hope that this post will incite a major internal witchhunt, which will ultimately destroy the Society from within. I pray that this occurs before I am found and "silenced."