When a EJB2.x MDB is deployed in JBoss4.x and 5, there are some additional (JBoss specific) MBeans registered. For ex: An MBean to stop/start delivery of messages to the MDB.
http://anonsvn.jboss.org/repos/jbossas/trunk/server/src/main/org/jboss/ejb/MessageDrivenContainer.java

// Try to register the instance pool as an MBean
 try
 {
...
 }
 catch(Throwable t)
 {
 log.debug("Failed to register pool as mbean", t);
 }
 // Initialize pool
 instancePool.setContainer(this);
 instancePool.create();

 for (Iterator it = proxyFactories.keySet().iterator(); it.hasNext();)
 {
 String invokerBinding = (String) it.next();
 EJBProxyFactory ci = (EJBProxyFactory) proxyFactories.get(invokerBinding);
 // Try to register the container invoker as an MBean
 try
 {
 ....
 }