Donovan has posted a preview of the managed OneNote API that I collaborated on. Donovan did most of the heavy lifting and is the papa of the actual Data Import API in OneNote. The managed interface that we worked on provides a very rich way to interact with OneNote using C# or VB.NET. This is what my and David's upcomming PowerToys are built on.
For anyone out there, I would love to see a faceless sys tray app that sucked my messenger history into a OneNote section. I don't have time to build this myself, but if you are interested in doing this download the files from Donnovan and start coding!