I have a MacBook Air and I have noticed that even though I have configured it to put itself into sleep mode after being inactive for 5 minutes it sometimes stays awake.
I've usually got a bunch of programs open (Tweetie, iTunes, Safari, iChat etc) but it there some way I can find out which program/process that is preventing the MacBook from sleeping? (Yes, I know that I could probably reboot the MacBook and try one program at a time but that is pretty tedious)