Activating Mountain Lion screen saver from Applescript, part 2
On Mountain Lion (10.8)

This AppleScript will start the screen saver with the selected folder.

tell application "System Events"
    set ss to screen saver "iLifeSlideshows"
    start ss
end tell


Lion (10.7) and earlier

First select the appropriate sources/folders by clicking the Shuffle item in the screen savers list, then checking one or more items in the dialog that appears.

Then start the screen saver as as in the Mountain Lion section, but change iLifeSlideshows to screensaver.shuffle.