Several people on the Apple forums have reported that the 'Lock Screen' function of Keychain Access doesn't work, despite having "Require password to wake this computer from sleep or screensaver" toggled on in the Security System Preferences panel.
I discovered that if I log out and then log in again, the screensaver lock works as expected, and remains functional until the next restart. FYI, my machine is started from cold every day, and Automatic Login is disabled. This abberant behaviour happens on my Quicksilver Dual 800, but not on my old and venerable B&W G3, both running 10.4.7.
Of course, YMMV ... hope this helps a few folks.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060724153822700