I have a note on "Using XSL as a Validation Language" at http://www.ascc.net/xml/en/utf-8/XSLvalidation.html also published in July Interchange magazine (ISUG). The implications of XPath are sketched out in the note "Axis Models and Path Models" at http://www.ascc.net/xml/en/utf-8/validaxis.html
Francis Norton has a subsequent article "Generating XSL for Schema Validation" at http://www.redrice.com/ci/generatingXslValidators.html which gives an XSL stylesheet for generating an XSL validator from a DCD schema. (Francis has worked further on this line, and may update that article and the software sometime, I believe.)
For anyone interested, there are also other articles on various alternatives to DTDs and content models at http://www.ascc.net/xml/en/utf-8/schemas.html ---Rick Jelliffe
A validator need not perform all possible validations to be still useful. I have a note "Weaker Validation" at http://www.ascc.net/xml/en/utf-8/weakvalid.html on this. In particular, for documents in development is it useful to have a weaker validation.