I have noticed that when i put my macbook to sleep while iTunes is open, and then awake my macbook after a extended period of time, my ATV will not show up in iTunes. So i started closing iTunes before i put the macbook to sleep and have not had a problem since. Before doing this i would have to restart the macbook and the ATV to get it to come back up. Now i just restart iTunes after waking up my MB. Give this a try if you are having these problems and then post here if it worked. If this does work for most people we can try to get it made into a sticky so that we dont have any more "help my ATV won't show up in iTunes" threads.