This is high on my own list as well. Some people I've talked to see federation and clustering as analogous. However, I've always thought it would be idea to provide clustering between several JMS servers (in other words, a queue defined on one server is defined on all in the cluster, etc.) as well as the support for federating the clusters into a network. In other words, you'd have a federated network of JMS clusters that could communicate via multicast or direct sockets.
Incidentally, don't you think JavaGroups might fit the bill to provide federation services much like it currently provides clustering services for JBoss?
I've thought about that as well. I thought they were going to go that direction with JMS 1.1, JCA 1.5 all under the umbrella of J2EE 1.4, but as far as I know they didn't make it. However, I don't see why that would rule out us going that route. I'm not that well versed in JCA, so I can't be sure.