The modern docx format (as well as OpenOffice's odt format) is just a zip archive of mostly XML files. Unzip the docx and xmllint the XML and you can get nice textual diffs. Rezip to go back to WYSIWYG.

I even wrote a simple tool to automate the process called musdex. It's on PyPI.

Even better: many useful "official" widgets are available as Open Source in the Silverlight toolkit:

http://codeplex.com/Silverlight

This toolkit is great even for WPF developers because they are liberally licensed (the MS-PL is like the BSD or MIT/X11 licenses) and thus quite customizable for WPF applications that need to do things that normal WPF controls can't quite handle.

Mono has had an implementation of the WPF subset most commonly referred to as WPF/E in Moonlight for some time. (1.0 is out there and very usable; 2.0 is in beta.) WPF/E is a strong enough subset that converting a WPF app may not be "trivial", it is doable, and it is "straightforward". WPF/E is also a "good standard" at this point because, in addition to Moonlight, Microsoft has took upon themselves to deliver official WPF/E support for Mac OS X (in Silverlight).