Here's a categorized Table of Contents to help you discover posts you might find interesting or relevant.
If you're new here, I'd recommend that you start reading at the top.
Why a New UI for Office 2007?
Overview of the New UI
Ribbon UI Elements
The Size of the Ribbon, Screen Real-Estate, Ribbon Scaling, and Minimization
Migrating to Office 2007
UI Themes and Visuals
Keyboard Control of the Ribbon
New Fonts for Office 2007
Customizing Office 2007
From First Sketches to the Final Design
An Inside Look Into UI Design, Usability, Development and Testing at Microsoft
New Features in Office 2007 Involving the New UI
The Office 2007 UI team