Clint,
I just re-read your post above about the animations already existing so why can't they make a SS out of it. Well, the greatness of Mr. Sachs' work isn't so much how realistic the fish appear, but more that they can look like that on your or mine computer. Compared to 3D objects in movies, the SS fish are mede up of amazingly few polygons (cue for Morgan to post the polygon count again) and still manage to look like the real thing.
Now a dinosaur is just so much more complex an object which might explain why Spielberg & Co didn't port their T-Rex to a ScreenSaver...
/Tiny Polygon Count
Imagine a "Steven Spielberg Screen Saver Studio", – Logical acronym SSSSS