With Tiger, you can insert Quartz compositions -- the animations created by Quartz Composer, part of the Developer Tools -- into a Keynote 2 presentation.
If you have the Xcode Developer Tools installed under Tiger, you can find some sample compositions at /Developer -> Examples -> Quartz Composer. For my own tests, I used the Slide.qtz example in the Parametrizable Compositions folder, and edited it to remove the text that sits on top of the color areas that slide around the animation field. In Keynote, use Insert -> Choose to add in the .qtz file. Once placed, Quartz compositions will look just like static images and can be manipulated as such. You can layer them with other elements like text, graphics, and QuickTime movies, and you can make them transparent with the Object Inspector, too, if you so choose.
In the full-screen presentation mode on a computer that can run Quartz compositions, however, the animations are no longer static. They play back, using the capabilities of your GPU, just as they would in the Quartz Composer tool. You can use this to create dynamic, animated backgrounds or other elements. You could also use compositions with slide builds -- but in my experience, the animations don't run until the build step affecting them has completed.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050708112631199