I just did a MSDN webcast on the OneNote API you can see it here:
MSDN Webcast: Developing Solutions and Extensions for Office OneNote 2007 (Level 200)
Just click on the "Register for event" button that is on the left hand side. Then login with your Passport/Windows Live account, go through a bunch of sign-in pages and then you can view the webcast. So I just tired this with Firefox and I would recommend using IE instead. It is shame that they don't detect the browser/WMP settings correctly. Maybe it is just my computer; in either event please check this out!