I know you can make your screensaver act as your desktop while you're logged in, but can you make your screensaver the background before you even log in?
I didn't even know you could change the background for your login screen, much less make it your screensaver. That would be cool, have the OS X login box imposed over the Matrix-style code running down the screen...
All as one line of course.
Then press ctrl-Z (animation halts) and then bg (animation continues). With ctrl-z you suspend it, so you get control back over your terminal and with bg you tell it to carry on in the background. AFAIK you can also run entire quicktime movies on your desktop this way or Quake / unreal sequences ... but I'm not sure how...
I don't know how you would go about to do that while not logged in / at the logion screen ... maybe set it on through remote login? I'll try something ... it would indeed be very cool!