You're right that a 60Hz display will get no benefit from 24Hz output, but it doesn't have to be 120Hz -- any multiple of 24 will do. Eg Pioneer plasmas have done 72Hz forever and in PAL regions there are quite a few 96Hz ones.
iTunes store content is mostly 23.976fps in my experience so 24Hz output would have made sense IMO. Most recent video SOCs have 1080p24 capability so there's a chance it might be enabled in a firmware update but because the ATV is basically an iPhone in a box, it might not have the same capabilities.
Interesting. I've been out in the boonies so I'm not quite up to speed on current tech. I thought the TV would have to have a common multiple for both 24p and 30p. How would a 72hz display it? Can it switch to 60hz like most PAL sets can switch between 50 and 60?