I'd reset the PMU also, since you didn't mention that. You've ruled out the Airport itself as the problem, which is good.
Take out the Airport, and with a flashlight, look down into the socket the card goes into. You will need to tilt the laptop upward so that you can see down into the slot, and it will still be difficult to see. What you are looking for are the two rows of metal pins...you want to make sure that all the pins are still straight, and that none of them have been smashed or bent (which can easily cause the problem you are describing). If you see any that are bent (usually on the ends), you will need to remove the top shielding in order to gain enough access to straighten them with tweezers. You can also just cut the visible section of the shielding off, although that is sloppier way to do it.
Also -- and this should actually be tried first -- as you know the Airport card plugs horizontally into a slot, and that slot itself is on a small card that plugs vertically down into the board. I've run into many cases in which the card gets slightly dislodged, causing the problem you've described. So, with the power off, use both thumbs to press firmly down on the area the Airpot card plugs into, in hopes that that will re-secure the slot itself to the board. Then re-install the Airport, cross your fingers, and power on.
The third possibility is that it's a software issue, so reinstalling the OS couldn't hurt, just to see.
The fourth possibility is that your board has gone bad in such a way that it can't see an Airport. I've seen it happen. But let's hope it's possibility 1-3. :-)
Hope that helps!
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