One of the first things that happens when a new version of Office appears is that people start to use its UI as a guide for their own applications. This makes a lot of sense because users of their applications are likely to also be users of Microsoft Office, and if that's the case then the users are more likely to instinctively understand the ISV's UI.
We've made this job a whole lot easier with the release of a License to use the Office Ribbon UI.
My favourite bit from Jensen's post is this:
If you choose to implement the Office UI, you sign up for the program by accepting the license terms and giving us a little bit of information about your product. There's no fee, you don't owe Microsoft any royalties, and the license is perpetual—meaning that the terms won't change.
Please don't hesitate to leave a comment here if you have created an application that uses this UI.