The long and the short of it, however, is that developers can use the Office 2007 UI (user interface) in open source projects as long as they meet the license terms. Not only that, the UI can be used on any platform.
As stated by Jensen Harris in "Licensing the 2007 Microsoft Office User Interface"
"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."Considering the high cost of development, I think this is both very fair and generous of Microsoft.
References provided by Jensen Harris:
- 2007 Microsoft Office System User Interface Licensing web site
- User Interface Guidelines Preview (A short excerpt of the actual guidelines)
- Channel 9 video discussion with more about the UI and the UI license
- Press release and Q & A about the licensing announcement
- Frequently asked questions