Microsoft made available OneNote Mobile on iTunes a little more than a week ago. Now your OneNote notebooks can follow you in your iPhone or iPod, wherever you go, as long as the iOS version is v.4.2 or greater.
Current version 1.2 brings some nice improvements, like shared notebooks sync, search, new color scheme and others (details on iTunes download page).
Mobile Notebooks are easily synced with their bigger, online-twin-brother, using OneNote Web App.
It would be great, though, if Microsoft developed an iPad version, which I would like to see in near future. In the meantime, grab this cute app here.