We're having the same problem on JBoss 4.2.1 GA - we compiled jbossweb 2.0.0.x and replaced the jbossweb.jar file under jboss-web.deployer as suggested, but getting the following error on startup. Are you sure no other change is needed?
org.jboss.deployment.DeploymentException: - nested throwable: (java.lang.reflect.UndeclaredThrowableException)
Caused by: java.lang.NoClassDefFoundError: org/apache/catalina/Lifecycle
It's odd, because Lifecycle is definitely found within the jbossweb.jar file.