I have a new iMac 27" and the air inlets are at the bottom and the outlet is in the middle of the back of the case, hidden behind the support arm. If you search for pics of the inside of a new iMac you can see that they did some awesome air flow design inside to pump the heat out without it sifting up through all of the non-heat generating components which could shorten their life.
You can see the outlet vent in the picture already posted.
Mike Sombrio wrote:
The exhaust vents are at the top, they can get very hot. I'd be sure to leave plenty of space so as not to restrict airflow.
I'm relatively new to the Mac platform having owned only a 2009 24" and 2010 27" prior to my current late-2012 27" both of which vented out the top which became very warm in normal everyday use.
This 2012, the "iMac13,2", is the coolest I have ever been around. The top of the computer, after having been on for weeks, is still at room temperature. Touching various areas around the back doesn't reveal any area that is more than very (very) slightly warm. I don't see how any space with any venting at all could be inadequate.