It looks like there's a redirection from
http://gwtproject.org/whatever to
http://www.gwtproject.org/ so you need to use
http://www.gwtproject.org/doctype/2.8.0/gwt-module.dtd rather than
http://gwtproject.org/doctype/2.8.0/gwt-module.dtd
But, really, if you ask me, you'd better not use a DTD at all. The only goal of the DTDs was to enable autocompletion in IDEs "for free" (without the need for a GWT-specific plugin). If you prepare for GWT3, you shouldn't have many things in you gwt.xml files besides <inherits>, <entry-point>, <source> and <super-source> (and some <define-property>, <set-property>, et al.; specifically, you should have fewer and fewer <replace-with> and <generate-with>), and that's not something you edit often anyway.