Yes Log4j is used in JBoss. For actual operational configuration look at JBOSS_HOME/server/default/conf/log4j.xml of the binary distribution.
For a starting point for the service in JBoss, look at server/src/main/org/jboss/logging in the source. You'll actually find many examples in the server branch of the source - hop into server/src/main/org/jboss/web for some usage.
Me stupid! I did a search through the source files using Microsoft's 'Search' utility. When I searched using FileLocatorPro I got a bunch of hits. I need to install FileLocatorPro at home too.