This happened to me on my 2009 MacBook Pro, I tried all the software fixes to no avail.
No WiFi Hardware Installed, Very frustrating, If you have a range of screwdrivers available and wish to
I decided to open up my MacBook Pro and investigate myself, if the problem was related to the Airport card,
On inspection after removing the screen to get to the area where the Wi-Fi card is housed, underneath the black plastic clutch cover, I simply resitted the cable where it connects to the WiFi card, I then had to re connect the airport card cable on the motherboard and the display connector and battery , before putting it all back together, I then powered up the laptop and instantly I had WiFi again.
There is a excellent video tutorial available on youtube:
http://youtu.be/cZnaF3e2S-Y
Good Luck.