I'm not sure if this is documented anywhere, but I found something interesting in the Sharing Preferences Panel. If you click on the text displayed after "Network Address:," it will flip back and forth between your IP Address and your actual Network Address. That is, 123.123.123.12 is originally displayed, and after one click, mymachine.mydomain.com is displayed. Obviously, your Network Address will vary.
[robg adds: In order for this to work, your IP address must resolve to a name...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030413153134259