Listener security guide
This is an excellent paper covering most of the basics needed to secure an Oracle listener. I had added a link to this paper to my site last year on my white papers page. There is also a useful tool from Integrigy that can be used to check the listener. I recommended this paper and tool to the person who emailed me and thought everyone else might benefit from this paper, either by re-reading it if you have seen it before (Like I did this afternoon) or to read it for the first time. It contains solid advice on securing an Oracle listener.