Apple is obviously aware of the Airport hack posted to MacBidouille/Hardmac last week. The hack showed how users of Core Duo Intel Macs could use the Airport card from the Core 2 Dup iMac to get up to speed with 802.11n.

In a report circulated by Apple today, this maneuver is relegated to the "how to void your warranty" file.
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).
So basically, you can't install it yourself under warranty, nor can an Authorized Service Provider install it for you. Buy a new Mac. ;)