Included with the license you’ll find the 2007 Microsoft Office System User Interface Guidelines. This 120 page document includes all of the information you need to implement Office-style UI; think of it as the specification for how the UI needs to work in your product. To stay within the terms of the license, you must follow these guidelines.
and it goes on:
The 120+ page guidelines document is confidential, so you’ll need to visit the licensing site and agree to a short evaluation license before downloading it.
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.