I have a customer willing to deploy a WAN-distributed JMS infrastructure with several hundreds of sites, business critical.
I am wondering whether open source implementations will scale at this level so I am looking for real world, big enough JMS deployments with open source products.

Note: beyond once-and-only once guaranteed, it is mandatory to tolerate the WAN failures, or you cannot call JMS a MOM. This means that the JMS server must implement a distributed storage for Queues and Topics, sometimes called "store and forward", as opposed to the most trivial central DBMS; this is not so obvious technically. Therefore, AFAIK, this is not so commonplace. It is also very difficult to get the information.
In the commercial world, with IBM, it is OK if you use WebSphere MQ alias MQseries (for a fee) as the JMS provider (they invented it!), but not if you use the embedded WebShere JMS (free). In WebLogic, at least 2 years ago, it was not OK. For SonicMQ or SwiftMQ or other pure players , I do not know.
Regarding opensource, JORAM from ObjectWeb does this, if you believe A ScalAgent Distributed Technologies White Paper. For the Jboss JMS, OpenJMS or Geronimo I do not know. Please help.

I am afraid that i only have worked with commercial products such as Software AG's EntireX. This product receives messages, saves then in a persistent store, and then is able to send the message to other processes that consume this messages. It is very useful to aliviate the work load of heavy used servers, because you can asynchronously send messages to other processes running of differents machines, and you are going to get the guarantee that the messages are never get lost. This could be useful for you, because if the WAN is down for hours (or days) you know that finally, the message is going to get to he receiver.

Jose Ramon Huerga
http://www.terra.es/personal/jrhuerga/

SwiftMQ is your friend. Look here for its Federated Router Network.

-- Andreas