If you have one of the new Mac Pro 2009 and would like to add an Airport card, you now can as it finally got delivered to Apple Centers under the reference number: ZM652-0999. However, you will not be able to purchase it and install it yourself. You will have to bring your computer to get it installed by a certified technician, making the cost of such installation much higher than the price of price of the card itself (45 €). As indicated on this photo originating from Macrumors forum, installation is tricky.
In particular the connection of both antennas is difficult. They have to be firmly inserted in a very small connector. As shown on the photo, this card is very similar to the model available in shops and that we used years ago to add 802.11n wireless capabilities to Core Duo MacBook and MacBook Pro.
