Anil Dash wrote an article for SixApart last year about using Movable Type to create a Microsoft Word document via WordML (download the schema from Microsoft).
It looks like the template on the SixApart website went missing. You can download the wordml.xml file it references here.
Ed Bott has a post about XML support in Office 2003 vs. Office 12. Anil posted a comment about Movable Type’s ability to create Word documents.