Hereafter is part of the internal note:
The AirPort Extreme Card sold for use in Mac Pro and some iMac computers is not supported in any other products and could cause damage if installed in them. Because of this possibility, the AirPort Extreme Card is not a Do-It-Yourself (DIY) part and requires installation by Apple Authorized Service Providers (AASPs).
Of course, it is not really surprising as Apple is probably not able to supply all requests for the Airport card model necessary for this upgrade. We strongly doubt that this card exchange might be dangerous, except if Apple did modify the PCI Express slot...
Instead of spending time and energy for preventing users to upgrade their machine to a new wireless technology, Apple should better fix the current issue affecting MacPro and iMac Core 2 Duo and limiting wireless data transfer speed to 512KB/s instead of the 8 to 9MB/s expected...
