I have created a little script that I use with GeekTool to show me my IP Address, what media is connected (WiFi/Ethernet), and my IP Address as the world sees me. The script is a culmination of some other hints and uses a program called setproxy, which I found on this website (original hint, setproxy's page).
I have it polled by Geektool every now and then, and I keep the script in an executable file in /usr/local/bin. I hope it helps someone out there. I'm not sure why, but the formatting doesn't seem to work in the Terminal window, but it's fine with Geektool.
See the script
Just copy and paste the above text into your favorite Terminal text editor, save it somewhere on your path, and make it executable (chmod 755 script_name). Then set up GeekTool to run the script (see GeekTool's Pick of the Week page for more info on the app).
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040930061422693