Microsoft Office User Experience Team Group Program Manager Jensen Harris has been focused on the UI model for Microsoft Office since 2003. At MIX08 he will provide never-before-shown early prototypes of the 2007 Office Ribbon. He will talk about the mistakes they learned from during the design process. He will walk through the principles of user experience design that lead to the creation of the first totally new user interface for Office.
Jensen's blog site features the The Office 2007 UI Bible which gives some great insights to the UI.