I created a small but useful bash script that simply takes a look at the network connection. It then speaks out an information message via the built-in text-to-speech system if the network isn't available, or if the connection is re-established.
I use it to see if the AirPort connection is lost, or if the network cable is plugged out. I run it on startup as deamon...
[robg adds: This worked as described for me, though I only tested it as a live script, not a startup task. Remember to make the script executable first (chmod a+x scriptname).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060110053733710