When I had the 24" LED ACD (before I sold it a couple months ago, I miss it so much!!), after using it for more than an hour (sometimes on for 8 hours or longer) the whole display would get pretty warm. I remember when sitting infront of the 24" sometimes you could feel a bit of warm heat from it but by no means it will burn you.
I think its pretty normal because the 24" Cinema Displays have a built in power supply instead of the outside brick like the older cinema displays. I remember on mine, the lower left fan would start to make a rattle noise after a couple of hours.. but not loud enough to be bothered by it.
I'm waiting for the 27" LED cinema display, please apple release it already!!!