An Omniscient (all-knowing) Being Does Not Have Free Will
If you are all-knowing, you know your future actions, what choices you will make, and you cannot change them otherwise your knowledge would be wrong, and you wouldn't be all-knowing. An omniscient being has no free will to choose actions; all its actions are predetermined.
"There is a light switch on the wall; God may either turn it on, or leave it off; but, since God already knows the future, God knows that he will turn it on. That is part of his knowledge. But what if God exercises freewill, and chooses not to turn it on. Is this possible?"
If you knew a decision you are going to make in the future... what would it mean? You would have no free will to change that choice. No option, no choices... based on the fact that you know its going to happen, it is predestined and no amount of strong will can change it. The further in the future the predicted choice is, the less free will you have to change it! Well imagine if for infinity you'd always known exactly what choices you were going to make, and that you could never be wrong. You would never have had any free will in any choice, ever!
In effect God is an observer. An omniscient being has no free will - its entire future is set out and it has no choice but to follow its predestined path.
http://www.vexen.co.uk/religion/god_has_no_free_will.html