I remember hearing/reading somewhere that a sensation of falling when you start to go to sleep is fairly common. I could remember any specifics about it though, so I went looking. What triggered my research is I seem to have had this happen a number of time recently and because it's followed by a jolt (almost to the point of bolting me into a sitting position) it's a little weird. It's not something to worry about, but still a little strange.
Anyway, it's known as a hypnagogic myoclonic twitch, though it's also called simply a myoclonic jerk, a hypnic jerk or simply a myoclonic twitch. Basically, is 'an involuntary muscle twitch (more generally known as myoclonus or a myoclonic twitch) which often occurs during the transition from wakefulness to sleep.' Experiences that occur when you are entering or leaving sleep are called hypnagogia.
Nobody seems to have a definitive reason as to why this happens. When you fall asleep your body goes through many changes; your muscles relax, your breathing slows, your heart rate drops and your temperature may drop. One thought is that the twitch (it sounds better than jerk) is a natural part of the sleep process (unlikely I think, I don't do it all the time, neither to most people), another is that the body interprets the signals of your muscles relaxing as you falling and tries to help you regain balance by causing the twitch, and a third, proposed by Carl Sagan is that this is a leftover from the days when our ancestors used to sleep in trees.
The fact that many people also experience hallucinations or visions of falling (dreams, by their normal definition, don't occur until you are in REM sleep), sometimes the sensation too, suggests that might be the most likely explanation.
Myoclonic twitches can also occur during full sleep, when you might not be aware of it, although this it typically called Periodic Limb Movement Disorder.
So, you see, nothing to worry about, it's fairly common, especially if you're tired or sleeping in an uncomfortable position. The body is a strange, complex and wondrous, if sometimes bizarre thing.
Sleep is very important people, aside from being one of the things that helps me fight off diseases better and stay healthier. Too much or too little has been shown to remove years from your life and, recently, researchers have found that sleep deprivation kills brain cells. Make sure you get the right amount whenever you can, I used to find meetings at work a great place to catch up on the odd 40 winks.