Go to the doc/samples/tomcat directory. Edit the build.xml file and run ant and you will get a Tomcat 5 configuration!
Assuming you 3.2.3...
Actually the directory is jboss-3.2.3/docs/examples/tomcat
Another way is to delete deploy/jbossweb41.sar and lib/javax.servlet.jar from your server instance and copy jbossweb50.sar from doc/examples/tomcat.
jboss-3.2.4RC1 uses tomcat5 by default now.