If you're technically inclined, you can actually write one without too much trouble!

Here's the popsquares source. Look how tiny it is!

GNOME Screensaver actually does support xscreensaver, so you can do this a few different ways :)

Here's a guide for writing an xscreensaver, and some examples, as well as how they're invoked by the screensaver it's self.

Here's a bit more from the GNOME website.

I'd personally love this app! Perhaps someone could write it!