Hi,
I am not sure whether this answers your question, but take a look at this page: http://download.oracle.com/javaee/5/tutorial/doc/bnaay.html
It explains how the different layers of a JavaEE application can be distributed to different servers.
Or do you mean JBoss specific clustering? http://www.mastertheboss.com/en/jboss-application-server/49-jboss-ejb3-in-a-cluster.html
Best regards
Wolfgang