What version of JBoss Portal? Did you install the Portal+AS bundle or if you installed JBossAS and the Portal separately, what version of JBossAS?
Have you changed the jboss-log4j.xml file?
Have you deployed a portlet that contains its own copy of the log4j jar file or it own log4j.properties file?
Finally, it would help if you posted the full log message for the error, including the 2 or 3 log entries before and after the error message - this would help to provide some context.
I tried this and I was able to skip that error.
I had to many different log4j.property files in my application which was being ported over from JRun.
hope this helps