I've been riding in clipless pedals for about two months now. I've fallen three times.
Once because I came to a stoplight and forgot I was clipped in.
Once because I was waiting for a car to clear an intersection, and balanced left when my right foot was the one I unclipped. I slowly, and embarassingly fell onto my left side.
The most recent, a few weeks ago, I unclipped my right foot, and put it down on a wet cobblestone crosswalk. I braked my bike to a stop... forgetting I had smooth and slppery cleats on. I came to a stop, my foot did not. As it slid forward, I went down with it, my bike landing on top of me. All in front of ten cars waiting for the light to turn.