iMac won't shut down per schedule Wife's 2007 24" 2.8 ghz 6GB RAM running SL, and now Lion.

For the past month or so, It won't automatically shut down at night per the schedule in System Prefs/Energy Saver pane.  My wife may not use the iMac for several days at a time, and then use it daily for a week or more - very random.

I've changed the settings by minutes, by hours.  I've Safe booted.

I will try logging her out and logging into my account tonight to see if it performs  there.

I do have a current clone and TM backups on an external drive.

Other than a complete reinstall of SL then Lion, what prefs are there and where are they that I could delete and re-establish to fix this problem?
Try an SMC reset and see if that helps.

You could also try using the pmset command in a shell (Terminal).