I don't remember Apple saying it needed to be in a open area. I thought they said you could have it inside an entertainment center with no issues. Personally, I would do it, but they said it doesn't hurt it.
I feel like many people have asked about the heat without understanding some basic physics...
The aluminum is hot because the whole case is one large heat sink.
You feel heat because the device is radiating heat, rather than trapping it.
The bottom grill of the AppleTV has holes for some heat dissipation, but you'll notice the drive is attached directly to the grill and the grill is attached directly to the aluminum casing.
One of the reasons Apple started toying with aluminum enclosures is because aluminum is an excellent dissipator of heat. Granted, the enclosure on the Mac Pro doesn't get that hot, but it has what.. nine fans cooling the internal temperature by expelling heat through the completely ventilated case?
The AppleTV has a bigger challenge because several fans would make the device noisy. So, naturally, the aluminum is leveraged even more here to do more of the work.
If the aluminum casing were cool on a barely ventilated enclosure like that, I'd be much more concerned because that means heat is NOT escaping the insides, overheating the circuitry.
That being said, the sides and top should have sufficient airflow/clearance to allow the heat to dissipate efficiently and to allow the antenna to be free of direct obstructions/interference.
tv, when I had it open to upgrade the drive from the 40GB to 160GB last June. Ran fine until last week when the 160GB started to make "interesting" sounds. So, EARLY Tuesday morning, I installed a new 250GB WD drive - in anticipation of the now-delayed update. Still have to get RMA for the 160GB, but the 
