There is a company called CambridgeDocs that has built a tool that will transform from WordML into XSL-FO. Here's a link to their site: http://www.cambridgedocs.com/products_wordml2FO.htm
This is just another example of a group that has leveraged the XML support in Office 2003 to build a tool to solve specific customer needs. As I've said numerous times, one of the big benefits we see with our XML fomats is that partners and other 3rd parties can now come along and build targeted solutions on top of Office. Here's a diagram from their site:
There definitely is a demand out there for tranforming WordML into XSL-FO, it just isn't a big enough demand for us to build directly into the product. Instead, as is often the case, we rely on 3rd parties to build these types of solutions, and by moving to a default open XML format, we make it that much easier for people to do just that. Remember if only 1% of our customers are in need of a specific feature, that's at least 4 million potential customers for a 3rd party to sell to.
I'm hoping to keep this blog moving forward with more and more information about how you can use the formats and help anyone that's interested in building a solution. The "Intro to Word/Excel XML" posts I made over the past few months are great starting points...