Comment Earth was temporarily hot due to a giant impact (Score 2, Informative) 135
The theory goes that a mars sized planetoid, named Theia, had formed at earths L4 or L5 Lagrangian points. As it's mass grew due to impacts, it was no longer stable at that point, and slammed into earth. The resulting debris cloud came together to form the moon.
See http://en.wikipedia.org/wiki/Giant_impact_hypothesis