My Mac has all of a sudden stoped going to sleep automatically! It can be put to sleep manually tho! 
TMA said:A number of things could stop it from sleeping. Check what programs you are leaving running, see if any of them are recording/playing/whatnot.
Counterfit said:First, calm down. Second, there might be a background process running somewhere keeping it from sleep. Have you checked the settings to make sure they haven't gotten borked?