Source for motion video backgrounds?

Digital Juice is the only one I use, though I use them in Final Cut Pro and hardly ever in Keynote. They should work, though you'll have to take the 720x480 and resize it in Keynote to fill the slide (make sure to unlock the proportions).

You can also use Quartz Compositions in Keynote, which offers more flexibility than pre-generated motion video if you're willing to muck about with Quartz Composer. (We've used this at work for a multimedia school show that runs partially on Keynote, and the results have been fine.) Some QC backgrounds can be found at QuartzCompositions.com.

I have the sense that, in many circumstances, using Quartz Compositions may require less horsepower from the computer (and thus yield better performance) than using pre-recorded video, but I'm not sure -- does anyone know?

PowerMac G5 Mac OS X (10.4.4)

I liked Digital Juice's product offerings, but they were a little too expensive for me. Animation Factory had a lot more useful things for me as a teacher; they also have quite a few straightforward (non-distracting) animated backgrounds in multiple formats: QT, MPG2, and WMV.