hmm.. can omniscence and omnipotence exist at the same time?
EDIT: my more eloquently stated reply to a similiar thread has been cut'n'pasted in substitution to the orginial reply. 
Omniscence would suggest that the future is pre-determined. If the future is pre-determined, then God cannot change it and is consequently not ominpotent.
Omnipotence would suggest that the future is not set in stone because God can modify it at will. If the future is changeable, then it cannot be known uphead of time meaning that God is not omniscent.