Via the IxDA list I noticed this item on the Jensen Harris’ An Office User Interface Blog explaining how application developers can license and implement the new Microsoft Office 2007 interface, “including the Ribbon, galleries, [and] the Mini Toolbar.”
Here are more details on the licensing process and the place where you actually go to download a license.
There is one fairly reasonable exception:
There’s only one limitation: if you are building a program which directly competes with Word, Excel, PowerPoint, Outlook, or Access (the Microsoft applications with the new UI), you can’t obtain the royalty-free license.