B2TR represents an iterative step forward for the UI design—a refinement and polishing of each component. The UI is now totally feature complete, and you will see only cosmetic differences between B2TR and the final version in most areas.
As I wrote in June, there are a number of changes in B2TR, ranging from minor tweaks to relatively significant improvements. In particular, we've made nearly 1000 individual improvements to the content in the Ribbon—everything from redesigning the Home tab of PowerPoint to subtle changes to scaling or labels to work better on small monitors. Most of these changes are, of course, very minor, but they add up to a new level of fit and finish in the overall user experience.
Tools and Utilities
2007 beta 2 technical refresh
Product page: here