Since I upgraded to iTunes 11, my iMac (27", 2011) goes to sleep during playback to my Apple TV. This didn't happen before the upgrade, so it has to be related to the upgrade. Does anyone what I could do to prevent this from happening? It's kind of annoying. Standard standby time for the iMac is 10 minutes. Thus, after 10 minutes of playback my iMac goes to sleep. Both, my iMac and my Apple TV are running the most up-to-date software.
I think the only way to prevent this from happening right now is to increase your standby time.
The reason it goes to sleep is that iTunes 11 does not set a "power assertion". Here's an Open Radar bug report on this issue: http://openradar.appspot.com/12791690.
Hopefully Apple will change this in an update because I'm having the same annoying problem.
Update 2012-12-13: iTunes 11.0.1 just came out and the missing power assertion has returned. Your computer should no longer sleep when streaming to the AppleTV.
I solved my problem. I simply restarted my Apple TV.
Despite the fact, that my problem occured after installing iTunes 11.0.0 (and later 11.0.1 the problem got solved by restating my Apple TV.