So the argument goes...
"An omnipotent god can create a being that performs an act known only to itself.
An omniscient god cannot do this.
It would appear, then, that no god can be both omnipotent and omniscient."
Hmmm... It sounds pretty good, doesn't it? Once you get over the shock-factor, though, it is nothing new.
To me, this is a repackaging of the old "If god is really omnipotent (all-powerful), can he create a rock too big for him to move?" The philosophical argument challenges what theologians call the "immutable attributes" of God, hence trying to show that they are illogical or mutually exclusive or ridiculous (and therefore that the conception of a 'god' is also ridiculous and illogical). The argument above seeks to find an intersection of incompatability between God's omnipotence and omniscience, His all-powerful and all-knowing aspects of His nature.
But let me be the first to admit some things...
God cannot lie...
God cannot choose to no longer be God...
God cannot do or be anything inconsistent with who He is.
That is basically what the argument in question is asking God to do -to create something He cannot know, to do something that is impossible. The implication is that "omnipotence" is defined as "God can do anything -everything is possible for God, even the impossible." I do not, however, believe that this is what "omnipotence" means, and I don't know of any thinking Christians who believe it does. I believe it means that God can do whatever He pleases, which is always things consistent with His vast nature. He cannot and will not do things inconsistent with His nature.
As C. S. Lewis wrote:
"His Omnipotence means power to do all that is intrinsically possible, not to do the intrinsically impossible. You may attribute miracles to Him, but not nonsense."
Or as Augustine wrote in City of God:
"For He is called omnipotent on account of His doing what He wills, not on account of His suffering what He wills not; for if that should befall Him, He would by no means be omnipotent. Wherefore, He cannot do some things for the very reason that He is omnipotent."
Is this just ignorance on the part of the atheist, or is it a straw-man argument -setting up a false representation of what the theist believes that is easy to tear down?
God knows... *wink*