it doesnt, it uses the case as a heatsync, so that it is silent and doesnt interfere with movie watching (like my noisy skyHD box) but there really is no need to actually touch an appletv as it has a remote and no on the box controls so it doesnt really matter.
Trust me, it has one. I ran my eSata cable right between the fan and the Power Supply plugin for the logic board out the back. In the link above [EDIT: two posts above] its the large black plastic square at the top left in the top picture. It is very quiet, but trust me its a fan.
However interestingly enough its on the bottom of the logic board between it and the base (picture is upside down) while the actual processor is between the logic board and the top of the atv with thermal paste between it and the top for enhanced heat conductivity just as others have noted. The top of the atv is indeed a heat sink. So while it has a fan, it does not directly push air onto the cpu as far as I can tell. At any rate, thats how mine is as well as every picture I have seen of one taken apart. Maybe your's is different ?
Note, it does seem that with no internal hard drive, the atv runs a touch cooler, which would make some sense. Though I honestly cannot back that up with hard numbers and is very likely the 'placebo' effect.