If you've validated your log4j.xml with its DTD (log4j.dtd) and there're still errors spewing out at the console of your application server, you can turn on debugging info using -Dlog4j at command line startup. In Tomcat just specify the following at the beginning of the catalina.bat file (in Windows):
At startup the console will be full of debugging output. It will show which log4j.xml file is read.
I found the culprit file which was residing in a JAR file :p...so I changed the file and updated the JAR file to make the errors go away.