Seems like classloading isolation in that EAR is not set correctly, otherwise it wouldn't find a different JBoss Logging jar. Check the AS5 docu further.
It's using the wrong version of JBoss Logging. I think you need at least JBoss Logging 3.x to get the Logger.getMessageLogger().
It was a combination of both. To resolved the classloading by updating the jboss-app.xml and specifying the following:
Also, I didn't have all the jboss logging related jar's (i.e., jboss-logging-3.1.0.CR2 AND jboss-logmanager-1.2.2.GA-redhat-1.jar).