I used to use SPDs and over a period of ~9 months my foot popped out of them twice when slowing down. Both times it was raining and I think that contributed. Finally, I adjusted the pedals...made them tighter...and it never happened again. I think its just an adjustment issue. You have to have them set tighter so its a little harder to clip in and out, then accidental unclips are very unlikely.
Now I've switched to various Crank Brothers pedals on all my bikes (mostly eggbeaters and quattros) and I've never accidentally unclipped.
I really liked the snappy, firm clip-unclip feeling of the SPDs. The clipping-unclipping action of the crank bronthers pedals is a little mushier, but overall they are a better pedal because they tighen as you pull up on them, making it impossible to accidetnally unclip without twisting your foot.