If you're using a decorator for the FacesServlet in GlassFish, you still have to provide a servlet entry for the FacesServlet due to an optimization in the container to only bootstrap a JSF application if a FacesServlet is present. No mapping is necessary.
[Message sent by forum member 'rlubke' (rlubke)]
http://forums.java.net/jive/thread.jspa?messageID=237422