As of WAS 7.0, you can now connect an MDB to a remote SIBus.
Great, so what does that mean? The Service Integration Bus is the feature as of WAS 6.0 that is a built-in JMS provider. Message-driven beans are EJBs for receiving JMS messages. For an application's MDB to receive messages from a queue in a particular SIBus bus instance, the application must be running in a WAS application server or cluster that is a member of the bus, basically meaning that the bus has one of its messaging engines running in the server/cluster. Thus when an MDB reads messages from a queue, the bus for that queue is essentially local to the application.
An MDB is configured by a JMS activation specification. The JMS activation specification in WAS 7.0 adds a new property, Provider endpoints, which (as the docs explain somewhat subtly) "allow the applications to consume messages from a remote cell." Technically, it will work with any remote bus, but the main reason to connect to a bus remotely is because it's in a different cell; if it were in the same cell, you could just add the application's server/cluster as a bus member of the bus and therefore make the bus local to the app.
With this property, when the MDB pool is activated, the beans will first try to connect to the bus specified in the Bus name property (if any). When that fails, it will then try to connect to the buses specified in the Provider endpoints. These Provider endpoint buses need not be local ones--ones where the server/cluster is a bus member. Because the buses are specified by the host name and port number of their bootstrap server (essentially, the point for connecting to a bus remotely via TCP/IP), the bus can be remote--the server/cluster does not have to be a bus member. A remote bus is sometimes also referred to as a foreign bus.
Normally to connect to a foreign bus, you need to use a service integration bus link to connect one of your server's local buses to the foreign bus. This is still the only way to send messages onto a destination on a foreign bus. But to receive messages from a destination on a foreign bus, you can now configure an MDB to connect to the bus remotely, bypassing the server's local buses (if any).
This new feature is now also available in WAS 6.x, but requires a fixpack. See PK77768: PROVIDER ENDPOINTS PROPERTY ON ACTIVATION SPECIFICATION NOT AVAILABLE WITH WAS V6.0/6.1 for more details.
Thanks to my colleague Rajesh Ramachandran for letting me know about this new feature.