This is one of several things that OWL DL oriented tools (eg. http://www.mygrid.org.uk/OWL/Validator) don't seem to like, since it mixes application schemas with the W3C builtins.
FWIW, the footer on the page of the tool mentioned [[ © University of Manchester, 2003, © University of Karlsruhe, 2003 ]] Do contemporary OWL DL tools complain about this triple?
