Hmmm, after examining my shoe I wonder if I in fact had a technical problem. The cleat had been rotated quite a way on the bottom of the shoe, even though the bolts were tight (I couldn't move it at all without loosening them, and I tried pretty hard). The rotation was in the direction you would expect if you tried to unclip but the cleat jammed in the pedal. So looking at it I wonder if I in fact that's what happened. I'll take a close look at the pedal before I go out tomorrow.