It depends so I didn't vote.

If I'm mountainbiking, it's whatever foot is necessary to keep me upright. You have to swing both ways on a mountainbike.

If I'm on the road, I clip out on the left and am righthanded. Reasons are:

1) Putting your right foot down (in the US) means the pavement is lower (sometimes much lower). It slopes away from you with the crown of the road. Between the slope and road debris, it's easier to have your right foot slip. I've seen it happen on tandems and I don't want to be one of the victims. I was riding clipless long before there were lugged mountain bike shoes, so bike shoes did not have any traction. Plus, you do what your tandem pilot does - same foot, period, end of story.

2) Chain grease. Like Michele said, it's hard to get grease on your leg if it stays attached to the pedal.