With so much negative talk here about Microsoft's speed & direction it's great to see gems like this once in while. I applaud the office team for reaching beyond Windows and providing people what they want and where they want it. With the PC landscape changing I think it's best for Microsoft to start targeting other platforms with their productivity apps. If they'd bring Microsoft development tools and APIs (read: .Net, Silverlight) to these other platforms they could also keep developers in the Microsoft fold. Locking this all into Windows is just shortsighted.
The office web apps are great too but as we saw on the initial iPhone everyone wants native apps. They're faster to get in and out of, are more fluid, and can provide a richer feature set. That said there will always be a place for the web apps as a backup when your principle computing tools are unavailable.
Oh, and of course in talking about OneNote it can't be left unsaid that OneNote ROCKS!