After applying the screensaver security update, my iMac 400 DV experienced sleep issues where it wouldn't go to sleep no matter what the Energy Saver was set to do. No applications were open, and the display would go black but the green light remained on all night. I did everything from reseting PRAM, running fsck, trying Norton, repairing permissions, open firmware reset, etc. Nothing worked.
The solution that I found was that to login as the root user and trashing the PowerManagement.bundle folder in /System -> Library -> System Configuration.
I then logged in via Ethernet to my iBook 800 and transferred a copy of the healthy folder onto the affected computer, changed ownership to "System," logged out from the root user, restarted, and then logged in as the regular user. I then tested everything, and it all worked fine, including letting it to go to sleep a couple of times without issues.
I guess it can work from any healthy Mac as long as its running the same version of the system. A person using the same fix but running a Power Mac G4 tower, did get initial positive results but then developed issues with display configurations on his dual display setup. Since my iMac has no specialty video cards, I guess it worked well.
[robg adds: I can't confirm this one; has anyone out there applied the same fix? Also, if you don't have a second Mac, you may be able to find a non-corrupted version of the PowerManagement.bundle by exploring the system restore disks...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030725235758270