I have recently been reading in Richard Dawkins' book, the idea that God being both omnipotent and omniscient is a contradiction. I think it is something along the lines of: if God is omniscient then He already knows how He is going to deploy His powers, which means He is effectively bound to act in a certain way -- meaning He is not omnipotent.
But I'm not sure I've totaly got my heads around the concept. Can anyone add anything more?
Many people think that God's having foreknowledge of my actions is incompatible with my acting freely. The argument you describe applies this reasoning to God Himself: His foreknowledge of His own future actions would render Him unfree. So either God is not free to do whatever He pleases (i.e., is not omnipotent) or God lacks foreknowledge. I find the original argument dubious. God's foreknowledge of my actions is not really incompatible with my freedom. For more on this, see Question 579 .