gnome-doc-utils CVS HEAD uses docbook2x-man. This is not provided by any fedora package i could find. On debian it is part of the docbook2man package.
What does it do, precisely? Convert what into what?
Convert a docbook into a man page. See http://bugzilla.gnome.org/show_bug.cgi?id=300697. It is used to generate the man page for xml2po.
Presumably you mean DocBook XML rather than DocBook SGML. Use 'xmlto man' for this.