http://www.macworld.com/weblogs/macosxhints/2006/01/lockscreen/index.php
"Just navigate to System -> Library -> Frameworks -> Screensaver.framework -> Versions -> A -> Resources, and then drag ScreenSaverEngine.app onto your dock, sidebar, or toolbar. Now when you want the screensaver to activate, just click the convenient icon. The dock will prove the easiest spot to reach, since its visible in all applications. If you have a launcher program such as Peter Maurers Butler or Objective Developments LaunchBar, you could even create a keyboard shortcut that will open the program for you, no mousing around required."
I personally use Quicksilver and assign "Command-L" as the trigger. Works like a charm.
Hope this helps.
-Todd