The default RDF/XML module is excellent, but not helpful for many Semantic Web users. The semantic web taxonomy standard in RDF is W3C's SKOS. SKOS is popular, many semantic web project support it, its the future, ...
We needed SKOS support for taxonomy_xml in the EU-funded research project "OrganiK" (http://organik-project.eu) and implemented it.
attached the skos_format.inc file embedded in a patch to now support SKOS.
we also had some changes to taxonomy_xml.module, but they are not necessary for you, anyway, registering the .inc should be enough (not part of the patch)
we tested it with various SKOS files and already fixed errors. It should work ... but not 100% sure.
we are interested to get this code into taxonomy_xml, but this is our first drupal.org contribution, so don't slap us too hard. I am willing to spare some time fixing bugs etc....
if there are issues, please bug me via firstname.lastname@example.org