Over at Mac OS X Hints a contributor has posted a hint with a script that will turn off Airport whenever the LAN port is used. For most folks this isn’t a concern but sometimes, in corporate environments, there are several reasons one may not want to be connected to ethernet and wi-fi at the same time.
The instructions are well explained though I had to reboot my system to get it working. In addition, you’ll also need to install growlnotify with Growl if you want to receive the pop-up notifications.
Updated 08/18/2011: I’ve been using this for about ten months and it’s really not an essential function. Sometimes it’s convenient and other times it can get in the way. In some situations when I wake my laptop up, after it was previously connected to the WiFi, it doesn’t automatically start Airport. All-in-all, it’s no better or worse than not having it installed. Your mileage may vary depending on your needs and network environments.