The icon is also denoted by the letter "N" which representative of OneNote, to enable users to easily identify the application. This will be consistent throughout all the new Office 2013 application icons. You can see this on the Excel 2013 icon below too:
A mysterious icon was accidentally leaked by Microsoft back in May this year on the Building Windows 8 blog. We can now confirm that the icon belongs to Outlook 2013. Check that icon in the screenshot below:
Update: More icons spotted:
Some more icons have made an appearance on the web recently, following Microsoft's TechEd EMEA 2012 conference.
Edited by SharmanNet, 05 July 2012 - 12:53 AM.