Apple - How do I prevent my iMac from sleeping while media is being played or streamed?

I experienced exactly the same issue. The steps to resolve the issue were the following:

• delete /Library/Preferences/SystemConfiguration/com.apple.PowerManagement.plist
• reboot

If it won't solve it another helpful step could be resetting the SMC: http://support.apple.com/kb/HT3964?viewlocale=en_US

For the general case, the easiest thing to do is not let your computer sleep. Go to Energy Saver in System Preferences and move the "Computer sleep" slider to "Never". If that's too extreme, you could move the slider to 3 hours which will keep your computer doing whatever it's doing for at least 3 hours after you leave it.

Either way, you can still let the display sleep to save power and the computer will look like it's sleeping.

Specific to AppleTV streaming, there appears to be a bug in iTunes 11. Here's an Open Radar bug report: http://openradar.appspot.com/12791690.

Update 2012-12-13: iTunes 11.0.1 just came out and the missing power assertion has returned. This should fix the AppleTV streaming problem.