OneNote SDK Now Live on MSDN
Here's something that was published on MSDN last week that you might have missed: the OneNote Software Developers Kit (SDK).
Because the developer functionality in OneNote SP1 primarily consists of the SimpleImport interface, the OneNote SDK provides a full schema reference. Each element and type in the schema gets its own topic, complete with a full description and usage example. So if you're looking for detailed definitions of schema elements, this is just the reference you want.
The SDK should be available soon as a download as well. I'll let you know when that happens as well. In the meantime, check out the online version.