Microsoft released the OneNote 2003 Service Pack 1 Preview today. Chris Pratley is a much better source for the intricacies. SP1 includes an API and SP1 will be the new OneNote baseline.
OneNote now has real-time note sharing. It would be great if we were able to discover available documents similar to SubEthaEdit instead of negotiating IP addresses. At least they did not require Passport logons.
You can also take video notes. My favorite feature of OneNote is the ability to synchronize your notes to a point in an audio recording. That ability is now expanded to video.