The one time I fell was something like you described, stick in spokes=bam land on head/back.

I couldn't see the being clipped in as being a factor, in fact I think that if I hadn't been clipped in the force of the fall would have thrown me further from the bike, instead I just landed on my back with the bike on top of me. My feet did come unclipped, and this was a fairly low-speed fall (I was going up a logpile and one of the logs came loose, so I was at the top of the pile at my lowest speed when the fall happened). Maybe some ladies could chime in with physics info, but I would think your trajectory would be the same in the situation with our without clipless pedals.

K.