James Clark released nXML, a new Emacs mode for editing XML documents (via Kevin Burton). The new mode provides validation and schema-sensitive editing using a Relax NG Compact Syntax (RNC) schema. You can convert DTD and Relax NG XML schemas to RNC schemas using Trang, another tool by James Clark.
Unfortunately, nXML does not seem to work with XEmacs yet. I'll take a look at it to see if it can be easily ported.