The problem with this question is that it assumes two things: That first,omnipotence is limitless power, and secondly, that just because God can, that He would. Both assumptions are false. First, let's examine omnipotence. If God is the source of His own power, then that power cannot logically exceed Himself. If it did, then the power would be superior to God. If God is the source of power, it comes from Him. Therefore, there's no such thing as limitless power. God limits His own power. To us it seems limitless because God can do what we cannot; create a life-sustaining universe. Second, if God can create such a rock, we have to ask why would He? What motive would there be to create something that cannot be used or interacted with in any logical or meaningful way? Therefore, God exercises logical self-discipline and sets an example for us all.