It’s a shame that the Mac OSĀ doesn’t have a “lock screen” function similar to Microsoft Window’s “Winkey + L”. One way around it is to set a password for when the screensaver is activated and then use the Hot Corners function and designate a corner of the screen to activate the screensaver.
Turns out that in Mac OS 10.5 Leopard there is a shortcut to activate the screensaver. Here is a tip from MacOSXhints.com, from the finder navigate System > Library > Frameworks > Screensaver.framework/Versions/A/Resources. Inside you should be able to find a file, ScreenSaverEngine.app. Running that will activate the screensaver immediately.
Unfortunately there is a slight snag with this tip. Apparently if you have it set to auto activate the screensaver at a preset time and you also manually activate the screensaver, the preset time screensaver will also activate at the same time as the already running screensaver causing some wierd video issues and also using up a lot of GPU. The recommendation is to create an Automator action. Steps how to set it up can be found on the second link above.