I have a 2011 iMac and have just upgraded to Mountain Lion. I'm currently using a USB keyboard and find that whenever the system goes to sleep the keyboard does not wake up. In order to get the keyboard to function I have to unplug it then plug it back into the port. As long as the machine does not go to sleep I'm Ok and everything operated normally. This problem did not occur with Lion and its only since I've upgraded that I have had this issue. Are there any ideas on how to fix this problem.