Unless you have configures JBoss AS to enable you to use your log4j.xml, your app's logs are governed by the settings in server/xxx/jboss-log4j.xml. If you have not changed jboss-log4j.xml, the logs for you app will appear in stdout and in server/xxx/log/server.log.
And one more thing, in AS 5.1.0, by default, only log entries of level INFO or higher are written to the log files. Your log entries are at the DEBUG level, so they are ignored.