I wanted to point out an update to the OneNote 2007 XML Schema. You can download it here: OneNote 2007 XML Schema which is part of the 2007 Office System: XML Schema Reference. You can still download the individual XSD or you can download this which will include other Office apps as well as having a better navigation experience as it is a CHM/Help file instead of just a plain text file.
If you all have questions please let me know. Thanks to Frank for getting this posted!