The common internet consensus is that Apple just swapped a couple pins. People have used non-airport cards in the airport slot... but they were all wifi.

I have not seen a pinout of the slot though, so I cannot really say any way or the other. If someone who knows what they are doing has a poke around perhaps more light might be shed. I do not know what pins wifi cards use and do not use.

It would be nice if some storage could be tossed into and otherwise useless airport slot though.