Yesterday I booted up my Dell Mini 9, and got the message "No Airport Card installed". Huh. It's running Mac OS X, but haven't had a problem at all. After trying a few things, and doing some searches, this finally worked for me (original thread at http://www.mydellmini.com/forum/dell-mini-9-os-x-discussion/...):
"you can try deleting com.apple.airport.preferences.plist, NetworkInterfaces.plist, and preferences.plist form /Library/Preferences/SystemConfiguration and restart to get the airport interface working again"
Voila! Problem solved.