One of the best features of Windows Phone 7, OneNote, and Skydrive is the ability to edit a notebook via the computer, web, or phone and have it automatically synchronize so that whichever platform you’re using, your notebook will be up-to-date.
A limitation when Windows Phone 7 launched was that you could only sync the “Personal (Web)” OneNote notebook that was created by default in your Skydrive “My Documents” section. This was troublesome as you may have existing notebooks syncing to other folders on Skydrive.
Nicely you can now open any shared notebook from your Skydrive by navigating to that notebook on skydrive.live.com from your phone’s web browser. Once you open the notebook it will connect it to OneNote on your phone and set up a sync relationship.
Found the info from Adam Lein’s post on Pocketnow.com