The contents of the queues will not be lost. This is the beauty of messaging and that's why it's being used on a such large scale. I've even seen posts on this forum where people said the only reason they like EJBs is MDBs.
The next time you restart the JBoss server your Message Driven Beans will consume these messages.
I am using JBoss 3.0 and inside the /default folder you can see a a /db/jbossmq/file/ folder and inside this folder you can see the folders for each queue (A,B,C,D,DLQ). If you browse into these folders you will see some files - these are the messages. In conclusion they are persisted on the disk until consumed.
Hope this helps.
