As mentioned on the Debian tracker (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=496069), Moodle currently includes a copy of domxml-php4-php5.php as part of the phpCAS which is embedded in /usr/share/moodle/auth/cas/CAS/.

That file is licensed under the Creative Commons "Attribution-ShareAlike 2.0 France" BY-SA (FR) license (http://creativecommons.org/licenses/by-sa/2.0/fr/). Unfortunately, this is incompatible with the GPL:

http://www.fsf.org/licensing/licenses/index_html#OtherLicenses

I have asked the author whether if he consider distributing this file also under the terms of the GPL, in which case, we could continue to use it in the CAS module.