Hi Steve,
By contents of the queue I mean just that, otherwise the persistence manager wouldn't have any problems with your queue. In short, if you are using Oracle for instance, you could check your messaging tables. This can be found in "jbossesb-server-4.9\server\default\deploy\jboss-messaging.sar" and checking the xxx-persistence-service.xml. That's the only way to be sure that there's nothing in your queue anymore.
You could also just drop your JBoss user (in Oracle at least) and set recreate tables in your xxx-persistence-service.xml to true. This way your tables will be recreated from scratch and you can find out if its a data problem or not.
Regards,
Hans