Locking Mac OS X Screen

From DuncanWiki

Just for future reference and any Googlers that may stumble on this thread: There are three ways to lock your screen.

You can open Keychain Access, open Preferences, and check the box "Show status in Menu bar". Then you will have a padlock in your menu bar that you can click to lock screen. (activates screensaver and requires a password)

Or, you can set up your system to require a password to come out of screen saver mode and assign a hot corner to screensaver. Then just flick your mouse to that hot corner to lock your display.

The final way is along the lines of what mercury was trying to do. Again, like #2 above, your screensaver must require a password. To activate your screensaver, just run the app /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ScreenSaverEngine.app.

FWIW.