i have the same problem and tried to configure the jboss as singleton. But it doesn't function. Looking at the "Singleton Message Driven Bean" in jboss.xml i found out that the both Configurations (single and standard message driven bean) are the same. The link in faq to the example configuration is dead. Do you have experiences with Singleton MDB?
PS: pleas take a look at http://www.jboss.org/index.html?module=bb&op=viewtopic&t=111562, where i described my problem.