One of the best resources to learn more about the new Office '12' user experience is definitely Jensen's blog. He has compiled a list of 30 postings that get you up to speed on the new UX. As a developer, there are a lot of exciting ways you can customize and extend the Office '12' user interface.

Posts about the Ribbon and how it works:

Posts about Galleries, Formatting, and Results-Oriented Design:

Why did we make a new UI for Office 12?

The philosophies, ideas, and design tenets behind the new UI:

Usability techniques and validation of the new UI: