Microsoft is making the Office 2007 ribbon UI available for developers royalty free. I'm not talking about the code, but the right to create a ribbon user interface in your own applications. There are a few things you do need to take into consideration:

1. You can't make a competing Office application.
2. You have to abide to a 120 page specification to make sure all applications work in the same manner (and sign an NDA to get this document).
3. Apply for the license which is perpetual once you get it.

Jensen Harris has an excellent blog post about it here. The Office UI Licensing page on MSDN is also a good starting point. And don't forget Infragistics has a ready made control library for .NET that includes most of the new controls (and you still need the license from Microsoft to use these).