Ah, you're making me engage gray matter today!!!!! Just for you though . . .

In general, the ball of your foot should be over the spindle of the pedal. According to Andy Pruitt, this works well for a men's size 9 foot. Longer feet may want the cleat farther back for stability and shorter feet will want it forward for the leverage. I wear a US women's 6 and run my cleats forward.

Ultra distance riders will frequently move their cleats back to solve hot foot problems because it takes the pressure off the ball of the foot.

Ok, everyone, run, don't walk, to your LBS (Local Book Store) and buy Andy Pruitt's Complete Medical Guide for Cyclists. It has a wealth of info for positioning.