The lead designer for the ribbon has this video from mix 08.

I really cannot recommend highly enough just how much you should watch this. Especially if you are using a ribbon control in your application. Jensen takes you on a tour of the prototyping the Office team did, consequently you know when to use the ribbon and in what context it is correct to do so. I have come across a lot of developers who have just used the ribbon ‘willy-nilly’ because it was the ‘new kid on the block’, and resulted in creating a really bad application, from a usability point of view. The ribbon solves one key user interface problem, and understanding what this problem is will result in you creating a better application.

There are lessons to be learned here that mean that even if you are not using the ribbon, just the thought process itself is sufficently edifying. It is fascinating just how bad the Office 2003 user interface is, and the scalability that the ribbon creates for your application. One is left beyond doubt just why Office 2007 is far better than the previous version, with an unimpeachable presentation from Jensen.