Microsoft has introduced OneNote Mobile app is for the iPhone and its available for download at the App Store.
OneNote Mobile allows you sync your notes online to your free Windows Live SkyDrive account so that you can access and edit them from virtually anywhere with your phone, PC, Mac, or web browser.
OneNote Mobile works on an iPhone or iPod touch with iOS 4.2 or above. The OneNote Web App also requires an appropriate device, Internet connection, supported Windows Internet Explorer, Safari, or Firefox browser and a free Windows Live ID.
Please note that there are some differences between the features of the OneNote Web App and the OneNote 2010 application.
The iPhone official description as of the time of this post, at the App Store is Free for a limited time.