By the way, upgrading to JBoss AS 6.0.0.Final would be a good idea.
Unfortunately changing the version as suggested in that link didn't help.
Please post the new jboss.xml that you are using and also the excpetion stacktrace (even if it the same as the one you mentioned earlier) that you see in JBoss AS 6.0.0.Final.
the xml header should be:
You will find the correct xsd (from which this header is copied) in "client\jboss-metadata-ejb.jar\schema\jboss_5_0.xsd"
Hope this helps