It is somewhat scalable, as the processing per message can be done across all your nodes. The limitation is that the node acting as the server may not be able to serve messages fast enough to the client nodes. This may scale well enough for your application, especially because message processing time is often quite a bit longer than managing the message itself. If a message takes 100ms to process by the client and 10ms to handle by the server (persist, send over the socket, etc.) you can have one server serve 10 nodes.
Unfortuantely, there is no active developer initiative at the moment to create a distributed solution, though JBoss 4 would likely help support such an architecture. Your thoughts are welcome on the developer forum.