We had a poll on this topic in August. There were 16 people who reported no tipover falls with clipless pedals, 29 who reported tipovers while learning, and 16 who reported occassional tipovers. So 26% of us converted to clipless padals with no tipover accidents.

http://forums.teamestrogen.com/showthread.php?t=9171

I've been riding seriously for 35 years, tipped over once ~32 years ago with cleats and toe clips, and never again. Been using Speedplay pedals for 5 years and love them. Used Looks for awhile before that. I've had plenty of close calls and panic stops where my foot barely came out in time, but somehow it always popped out. Either luck or a panic pull that brakes it loose.