Got this tip from a comment here => http://www.gkoya.com/2006/11/23/locktight-for-mac-os-x-intel/
As of Mac OS X 10.6 Snow Leopard, there is one more way to accomplish this, without any extra or third party software.
Use Automator to create and save a ‘Start Screen Saver’ service that accepts no input and is available to all applications. You can find the appropriate Automator action under ‘Utilities’. Then you can simply assign this new service a shortcut from the Keyboard Shortcuts preference pane.
If you would prefer to lock the screen rather than use a screen saver, use Automator to run a shell script with the following command:
/System/Library/CoreServices/Menu\ Extras/User.menu/Contents/Resources/CGSession -suspend
Then set the keyboard shortcut as before. Huzzah!