A DTD is a Document Type Definition. A DTD describes the legal structure of an XML file.
The DOCTYPE declaration in an XML file usually contains a URL. By convention this URL can be used to retrieve the dtd file, but the XML specification (actually SGML but that is a another story) does not require that the URL be a valid address.
The reason there is nothing at that location today is because the build script for our web site is currently broken (marc is working on it today).
Thanks for your information!
I don't know what I was thinking when I wrote that post.
The dtds are in the docs/dtd directory of the JBoss 3.x distributions.