A variety of free and commercial tools exist for doing "up conversion" of non-XML formats to DocBook. The SGML Buyer's Guide calls these types of conversion tools N-converters (short for "Non-SGML/XML source document converters").
Html2DocBook, converts XHTML
txt2docbook, converts plain text
In http://mail.gnu.org/pipermail/help-texinfo/2002-December/000851.html is described a method for going from latex to Texinfo. Then you can use the above two tools to get a transformer from latex to Texinfo.
upCast converts Microsoft Word documents to XML. It comes with a filter (an xslt stylesheet) that outputs the document as a DocBook article. upCast is commercial but they do offer a free version for non-commercial use. The free non-commercial license is somewhat hidden on their website. Go to the Licenses page and scroll down to the "Private Licences" heading.
Wordplay is a Word-to-XML converter. Comes with an XSLT stylesheet to easily export Docbook-compliant XML from Word (commercial).
YAWC is a plug-in for Microsoft Word (97, 2000 and XP) that can generate XML according to any XML DTD. YAWC ships with support for the Simplified DocBook XML DTD, but any other DTD can be supported with a small amount of configuration work. Pro (commercial) and Lite (free) versions available.
x4o is a plug-in for Microsoft Word (97, 2000 and XP) that can generate XML according to any XML DTD. Commercial.