I think the intent is for Core Image and Core Video to gracefully degrade -- if your application calls Core Image, but the hardware doesn't support it, the app won't break, the effect just won't appear.
So I doubt that a Core Image-enabled version of Keynote wouldn't run presentations on lower-end machines -- my guess is that, instead, the transitions and effects that depend on Core Image simply won't work. That's not ideal, but it's better than a) having the entire app break, or b) waiting to incorporate the cool Core Image technology into Keynote until eveyone has machines that will support it.