Standby during playback since iTunes 11
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.