[baudrunner]

If you picture the earth as a ball rolling in its path around the sun then it makes perfect sense that the rotations coincide because the ball is rolling forward and moving forward as it is doing so. Both the orbital direction and the spin are always in sync in a forward rolling object. All the earth is doing is rolling around in elipses.

There are exceptions to the rule, in fact there is/are more than one orbiting bodies in this solar system whose spin is counter to the direction of the orbital path. One is a moon around Saturn, I forget the name, too lazy to look it up. The experts conclude that they were thrown from another body and haven't had time to recalibrate. They should pose interesting environments for study.