Наоборот, в Open Source использовать можно: «You can use the UI in open source projects as long as the license terms are consistent with our license. You can use it on any platform: Windows, Mac, Linux, etc. If you're an ISV, you can build and sell a set of controls based on the new Office UI.»
Microsoft лицензирует интерфейс Office 2007 сторонним разработчикам