I'm confused.. how is this Apple's Javascript library? It was not written by Apple and the author (and his company) don't appear to have any affliation with Apple..
The author of Sproutcore is also the head of .mac UI development group at Apple. He had a previous startup (which might still exist?) that sold a corporate webmail product, built on top of Sproutcore. Apple hired him to work on .mac. Sproutcore is what's underneath the apps in Apple's forthcoming MobileMe service.
I'm really not that impressed by it. There is nothing there that is any more impressive than Dojo, MooTools, or jQuery.