if you're going the SPD route, make sure to note Shimano has two models: fixed or floating (I *think* that's what they're called). The floaters allow for a small amount of lateral pivot, and will not only disengage with a regular sideways twist, but also if you pull up on the pedal really hard. The fixed ones don't allow anything but a lateral twist.
If you're concerned with (hee hee) accidental release, go for the fixed (or non-floating, whatever they're called) cleats.
AG