I have an Airplay-capable (mid-2011) iMac running ML. Airplay, though, only works intermittently, with the icon disappearing from the menubar for no apparent reason. When this happens, I check the Displays pref pane, which says "Airplay mirroring: No devices detected." However my AppleTV (2nd gen) is on and connected to the wifi network. It shows up as an option in iTunes and shows up in Airport Utility. So it's there and it's on and receiving, but Airplay doesn't "see" it for some reason. Then, randomly, it'll pop back up later. AppleTV is connected wirelessly. iMac is connected by ethernet to the Airport base station. Any ideas??? When the icon is in the menubar, everything works perfectly. But it just vanishes for long stretches of time for no reason!