Looks like the AppleTV appliance gets pretty hot while in use. The DigitalTrends guys used an IR cam to capture the heat signature. I think this last quote sums it up best:

"To echo my first review, be sure to keep the Apple TV in a well ventilated area and please, please do not set anything on top of the Apple TV whether it's in sleep mode or in full operation. The thermal insulation of another object on top of it could prove damaging."