Just recently, my MacBook Pro (Mountain Lion 10.8.4) will no longer go into sleep mode. When I press Cmd+Option+Eject, the screen goes black for just a second but then comes back on. If I close the lid, the screen turns off but the system remains on.
I'm pretty sure this started after running uTorrent, which has an option to prevent sleep while a transfer is running. However, I've disabled that option now, but the system still won't sleep.
When I run
pmset -g in the terminal, there is a line that says:
sleep 120 (sleep prevented by )
I think there is supposed to be a PID listed there, but it's just blank, which makes me wonder if I've hit some bug that corrupted a system setting that needs to be reset.
Sleep is prevented after a clean boot, with all non-system disks ejected, no external monitors, with no third-party programs running (even Dropbox and Evernote daemons).