Looks like some more documents on what’s new in OneNote SP1 are now online:

OneNote 2003 Service Pack 1 Overview
OneNote 2003 Product Guide
Service Pack 1 FAQ

I’m not sure if everyone noticed this or not, but check out the last bullet in the overview:

Simple application programming interface (API) for extending OneNote. Third-party developers can build OneNote add-ons to import new data types into a user’s notebook, enabling integration with other products and devices.

Now the fun begins.