Again, this wold mainly help in development and very limited production sites. If I'm running a bigger site or a cluster than I can tell it to use a database. I think this is pretty much a given at this point. The JTA enabled MQ JMS would be so much more useful if they added message beans at the same time. This is the main feature I'm waiting for from the EJB 2.0 specification.
Clients send messages but servers usually both send and receive messages.
Once I receive a message from JMS, I want to be able to authenticate as the user who sent it as my message beans authentication. I went to the trouble of setting up ACLs on bean methods etc and if I always run as the same user when processing incoming messages then whats the point on using security at all frankly. This pluggable mechanism may be as simple as saying to WAS that I am user X after extracting X from the message payload of a header. WAS then trusts me that the message came from that user. Obviously, if we don't strongly authenticate the message payload then this is a big hole.