Somewhere around here I have an original Airport with similar symptoms.

In my case, the card is definately detected by OS 9, X, and Linux. It actually seems to work for a short time after power-on, then goes effectively deaf. At that point, Tiger and Leopard both tell me that the card is off, but trying to turn it back on does nothing.

In my case, I suspect that something has failed or is failing in the radio section of the card causing the radio to stop working once the card warms up.

JR