Simple question:
In JBoss 5.1, what JAR do I need to add to the classpath in order to use org.jboss.annotation.ejb.Clustered?
That annotation isn't relevant for JBoss AS 5.x. Those org.jboss.annotation.ejb.* have now moved to org.jboss.ejb3.annotation.* and reside in jboss-ejb3-ext-api.jar (Found in JBOSS_HOME/client).