started to have this behaviour after one of these system upgrades from 10.4.x to 10.4.y. Can't tell anymore if it was PPC only or Intel as well.
It sometimes also wasn't possible to lock the screen with a password secured screen saver. It occurred with all users (including root or newly created testusers), after moving or deleting whatever preference file there was and even after a PRAM-reset.
It could sometimes be fixed with a reboot. Very often the problem stayed even then. Always working was to reboot, to login, to check if the screensaver asks for your credentials or not and if not to logout/login.

For me eventually working was deleting these files in /var/vm/app_profile which seems to be some caching mechanism to speed up the boot process.
If the problem occurred, booting into single user mode, letting the applejack script (see applejack.sf.net; maybe any other maintenance tool will do) doing its job, rebooting the machine and everything was fine again.

no experience with 10.5.x but I think this directory doesn't exist any more (?)