as far as cluster-enabling a Seam (EJB 3) app, all you need to do is the following:
1) add to web.xml
2) add @Clustered to your stateful session beans
you can also configure your entity classes to be cached using Hibernate 2nd level cache.
But as long as you do the two steps above, you don't need to worry.
Keep in mind that if you're using mod_jk with IIS or httpd, make sure that sticky_sessions is enabled (this is a configuration that is highly recommended by JBoss). see Tomcat Connector info on JK.
For MDB's deployed in the all folder you don't need to do anything: they find the destination and subscribe, whether it is remote or local; and when the coordinator-node dies, they resubscribe as soon as the new coordinator-node is ready hosting the destinations. So this makes me believe that the @Resource ConnectionFactory injection is cluster-aware somehow.
For client side lookups you should use the right jndi parameters and port 1100 instead of 1099: this will make sure the client-side proxy finds a working node.
For @EJB i dont think this would make sense since it meant for injecting local dependencies.