Dave Johnson discusses the problem of getting a WAR file to work on JBoss 6; he ended up producing a separate WAR File for that server platform.
I have come to view WAR files as per-installation artifacts, you will have different ones for different clusters, so rather than be surprised by this, have a build and deploy process that generates the right one.
Even with this death of the "one WAR-file, anywhere" story, it's still a good single artifact to push out. Along with the updated JAR files to stick in your app server class path. And the environment variables needed to tune the app server...