A story I heard about 2001 was that they were using Blue Danube to edit to and Kubrick liked it so much that's how it stayed.
The way I remember it was that MGM was getting worried about the film. Kubrick had been working on it since 65, so they needed a progress report so to speak. So Kubrick finished off a rough cut (for MGM) using music from his personal collection, and he liked it so much it stayed. Or something like that.
GoodMusician, I've always wanted to get my hands on that Alex North score. I'd heard that it was officially released, but I can't seem to find it. Do you know if the release is out of production or something?