Ever since I installed the 10.1.1 update on my PB (G4/400, 384 MB RAM), I noticed this weird problem.... When I was logged in (plain vanilla admin type account, NOT root), the system would go into sleep mode after 10 minutes, the amount I set in the Energy Saver prefs panel. No problem. But when my wife was logged in, it would never sleep! (It went to sleep for both of us just fine using 10.1) A peek at her Energy Saver panel showed the same settings that I had established, except they were dimmed, meaning that she couldn't change anything without an admin user & passwd. OK, so I "clicked the lock to make changes," supplied my admin user/pass, moved the sliders around a bit, eventually setting them back to where I wanted them (and where they were originally). Quit System Prefs, and wait for the system to go to sleep.... Never happened. :( The backlight shut itself off, but the HD didn't spin down, and the sleep light never started pulsing.
The only way I could get the system to time out to sleep while she was logged in was to copy the com.apple.PowerManagement.plist file from /Library/Preferences to the Library/Preferences directory in her home dir. After doing that, the System Prefs window for Energy Saver still looked exactly the same, but the system now goes to sleep for her!
I don't know if this problem is at all common -- I couldn't find any messages anywhere I looked (on several discussion boards) about this particular problem. But I'm glad I found a solution!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011211111946248