XHTML as the format: Why? - All Wikis can produce something approaching XHTML. It's relatively straightforward to generate any Wiki syntax from XHTML.
UniWakka exporting to desktop word processor and XML standard
Uniwakka exports to OpenOffice .sxw format which is a potential international XML standard. The sxw file is actually five files zipped together - content.xml, styles.xml, meta.xml, mimetype and settings.xml
At the moment, it means you can see all your wiki pages in OpenOffice. soon you should be able to import them and edit them. Perhaps you might also be able to save your word processed documents as a ready made wiki.
Uniwwakka is here: http://uniwakka.sourceforge.net/HomePage
Uniwakka also has great database, biliography, footnote abilities. It uses Bibliotex to create bibliographic records or even sophisticated commented bookmarks to include in writings, publications or to store. It's pretty easy to use: http://www.istitutocolli.org/uniwakka/BiblioInsert