Microsoft has just released a OneNote app for the iPhone, and it's free "for a limited time".
As with the Windows Phone 7 app, the iPhone app will let you manage notes and shopping lists (and even add photos taken with the iPhone's camera), and then sync those with Windows Live SkyDrive so you can access them in either the Windows desktop application or its web-based counterpart.
The OneNote app is currently only available to US users. We are still waiting for reports on the app's accessibility.
Old and now out of date news