I just got this question via email and realised I don't know what the state-of-the-art answer is : given some XML with DTD/XML Schemas, how best/easiest to convert it to RDFS/OWL?
Seems like there should be some schema translation tools by now. Ok, ok, I know the syntax layer of XML schemas doesn't naturally map onto the semantic layer of RDFS/OWL, but usually the XML structural pieces do correspond to entities and relationships, and some automation should be possible at least for the first pass.
Here's what I've got so far: I've generally used XSLT, usually created manually but there are some notes re. tools that can be useful here (Sun's RELAX NG Converter, Relaxer and Trang. The W3C RDF validator is very handy… Bob DuCharme has done " A Somewhat Generic XML to RDF Converter" and if the XML is close to what the stuff might look like as RDF/XML it might be possible to tweak it along the lines of Make Your XML RDF-Friendly (Bob again, with John Cowan).
A trick for when the XML is close to RDF/XML already was
(thinking Atom/RDF), add bits to the DTDs
to fill in fixed default attributes, e.g.
<!ATTLIST atom:entry rdf:parseType CDATA #FIXED
So an DTD-friendly XML parser would see the missing bits. No idea if it works ;-)