Jensen Harris from the Office 2007 design team gave a very rare presentation at Microsoft's MIX08.

Farewell, menus and toolbars! Come hear the behind-the-scenes story of how the 2007 Office Ribbon user interface was imagined, designed, and validated. See never-before-shown early prototypes, hear about mistakes we learned from during the design process, and find out the principles of user experience design we used each step of the way in creating the first totally new user interface for Office.

Portal:
http://www.babybluebox.com
http://www.creativecrew.org
http://www.williamchang.org (less info)