I'm sure you have probably seen that the
OneNote SP1 preview is available. I downloaded and installed this today and I must say that I like what I am seeing. I use OneNote quite a bit anyway, but they have added some really nice features.
This page outlines some of the changes. Some of the ones that I immediately liked was the better integration with Outlook, the ability to send notes from my Axim PocketPC directly to OneNote and of course there is now an API to play with.