Here's a pretty good explaination of the phenomenon:
Fixed link, and the hypnic jerk
I notice I tend to do that alot if I've over-caffinated too close to bed time, or if I'm stressed out for one reason or another. Unfortunately, because I understand the phenomenon, I no longer have the "falling" dreams.
While uncomfortable, and startling, it was at least better than snapping awake, and bitching because I made the mistake of drinking coffee two hours before I went to bed. :P
Kinda takes all the "magic" out of it, you know?