The ScreenSaverEngine.app that is located in the /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ directory can be used to invoke the screen saver on Mac OS X. So to do so from the command line you would just use the following:
open /System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ScreenSaverEngine.app
