Chris Pratley has a write up on OneNote SP1 Preview. This is a huge upgrade and has lots and lots of new features. I've been running it for a few months now and love it.

Here are my favorites:

1. Capture Screen Shots and Snippets.
2. Scratch Out Gesture for Ink.
3. Password Protection of Sections.
4. Insert Meeting details from Outlook.
5. Data Import API (I am writing two of the PowerToys that Chris talks about) ;-) and collaborating on a nice .NET managed API that hooks into OneNote's API.
6. Lots and lots of Inking improvements. Inking feels much more natural.
7. Insert pictures and documents.