About a month ago (in July 2010), four people collaborated with Goolge to determine the fewest number of moves to solve the Rubic's Cube. God, being omniscience, already knew this. Thus the number of moves that He needs to solve the Rubic's Cube is at most 20.
Personally, I find it rather odd that this number is called God's number and that the algorithm used is called God's algorithm. It is just another way of saying that this is the optimal solution. In computer science, we talk about optimal solutions all the time (at least up to constant factors), but we never use this term.
(All I am saying is that) this is just the only use of the phrase as far as I know. Oh well.
God's Number is 20