Does this smell like embrace, extend, extinguish to anyone else?
Things that have received a lot of attention in the past year or two are Open Office / ODF, Google search and gmail, and Firefox. Obviously, if the trend continues and more people move to a non-MS browser and non-MS apps, the possibility for users to choose a non-MS OS increases.
My company uses Novell Groupwise for email, and one of the biggest complaints of new employees (who are pretty much all used to Outlook) is that they can't seamlessly use Word as a mail editor. Obviously a great deal of HTML mail is being created in Word, so what better place to render it than Word? This would allow MS to make changes in Word's HTML engine that would render fine on another MS box with Outlook/Word, but not on other platforms.
MS was blindsided when it came to the ways we would use the web, and could use this as a way to steal it back. With Word's built-in HTML engine, they could potentially do away with IE and make Word the web browser. For MS shops, this would have the added bonus of allowing them to use a web repository for document storage and Word as the editor/browser for massaging docs, and over a period of time discontinue support for old-fashined HTML. With OOXML being pushed by MS as the new "open standard," MS will have a hiding place from anti-trust problems, and OOXML could forcefully become a de-facto standard for email and the web.
Google search - replaced by Ctrl-F
gmail - Google could pay to have an OOXML parser written for their needs, but would they?
OpenOffice - Improvements to the software will take a backseat to implementing a not-perfect OOXML filter.
Firefox - Battling Word instead of IE, but due to the same reasons.