Since we got my mom's powermac, I noticed something odd about it -- every now and then it'd sit with the display on for ages and ages, and then I tried to sleep the machine, it wouldn't sleep!
No, nothing's playing in itunes, the idle-sleep is set, and the idle-display sleep is set, wake for network activity and power failure are off. The machine's plugged into the network via ethernet, optical audio for sound, ADC for display, and a bluetooth keyboard/mouse setup..
I have a feeling it's a "normal quirk" for one of the above, but if it is, that's insane. Personally I'm leaning towards the video card, but have absolutely no idea...
Two more things: Sleeping the machine from the menu bar in that state causes a spinning beachball of death, and in normal situations where you have regular audio-out enabled (which it keeps reverting to), it STILL outputs to the optical out.....
Perhaps the logic board? I dunno.. The hard part is we've got no way to take this into a service place right now.