The attached XSL will convert a MM file to TSK, the format used by TaskCoach ( http://www.taskcoach.org ), a popular todo manager (FOSS).
At the moment it doesn't export richtext nodes properly -- surely you already have a good way to "flatten" them, but I can't find it. TaskCoach doesn't handle HTML, afaik.
TSK files require a date-time value in the "startdate" attribute of tasks; it has to be set in the past for nodes to be seen as "active". I don't know if your XSL parser handles XSLT 2.0 (many don't), so I used a 1.0 extension available online in order to do that -- I add the current timestamp, and since more than a second will always pass between the file being saved and the export being opened in TaskCoach, it seems to work.
Let me know if you need anything else.