WYSIWYG Editor for DocBook DTD Content?

Saqib Ali asks: "This week I saw a demo of the Tagless Editor by i4i. The editor is a plugin to Microsoft Word, which can be used to create XML based content. The plugin can handle various custom DTDs. However it can not properly handle the DocBook DTD. I was wondering if there is any WYSIWYG XML editor that can be used to edit DocBook DTD based content? Any ideas?"