So... Google is your friend...
And Bing wants to be but it's like that friend that you call when your friends are all out of town or doing something lame.
2nd link that Google pulled up for me when I googled:
"movies screensaver mac"
http://www.apple.com/downloads/macosx/icons_screensavers/moviescreensaver.html
Simple AppleScript/Automator action should allow you to play an iTunes playlist when screensaver starts...
I'm awfully tired or I'd scrounge the corners of the web looking for it for you...
Godspeed.