If God already knows what we shall pray for, then the prayer is unnecessary. Of course, as I replied to simmania above, go forth and demonstrate this power of prayer, and you can save all the stubborn atheists who yearn for a physical proof of God. It would take only a few mountains move into the sea at the request of a petitioner to convince them.
I am in a playful mood, so I will direct your attention to the definition of prayer in Ambrose Bierce’s immortal book: “The devil’s dictionary”. “To pray (verb): to ask that the laws of the universe be annulled on behalf of a single petitioner, confessedly unworthy”. Pretty succinct, wouldn’t you say?
But, jokes aside, if God’s knowledge is contingent upon what we shall ask for, and if God is simple, meaning that God’s knowledge is integral part of his essence, then God’s essence is contingent upon our decisions to pray for something. Not a good prospect for a “necessary” being, whose essence is contingent, eh? (Which makes omniscince also meaningless, next to immutability…)