I had this problem after installing Forcedeth.kext for ethernet. I had to go to /Library/Preferences/SystemConfiguration and open up NetworkInterfaces.plist. Change it so that your ethernet is en0 and your airport wireless is en1. Save it and reboot. See if that fixes the problem.
If that doesn't work, just delete NetworkInterfaces.plist and reboot. After you reboot, go to the Network pane in System Preferences and it will say it found a new network interface. Click apply and wait a few seconds. You should be good to go.
Great Answer !!!! I had this problem after upgrading my motherboard and RealtekR1000 kext. Deleting the .plist worked for me.