The iBook was bumped to 900MHz and a 60GB HD. No other changes seem to have been made (no Bluetooth, for instance), so it's most likely a bulk component adjustment.
Piles of stuff
Mac OS X 10.3 is supposed to include a number of UI enhancements, most notably the concept of document piles (which seems pretty neat - let's wait and see how it's actually implemented).
In the meantime, Clutter has implemented the concept for playing CDs in iTunes by simply clicking on cover art pictures: