MM, exactly which "dual G5" do you have - dual processor or dual core? There is a key difference:
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To get the full speed benefits of the 802.11g-based AirPort Extreme Card in your Apple computer, you’ll want to use it with an AirPort Extreme Base Station. But since the AirPort Extreme Card is compatible with 802.11b, you can use any AirPort Base Station or other Wi-Fi certified 802.11b access point without any hassles; you don’t need to change any hardware or software settings. Everything is automatic.
To create your own wireless network in your home or business, just add an AirPort Extreme Base Station. This will allow you to communicate between AirPort-enabled computers, AirPort Extreme-enabled computers, Windows PCs with Wi-Fi certified 802.11b cards, and the AirPort Extreme Base Station and share access to the Internet, without wires.
System Requirements:
The AirPort Extreme Card can be installed only in computers that are AirPort Extreme ready. AirPort Extreme Cards cannot be used in AirPort-ready computers with an AirPort Card slot.
Requires Mac OS X 10.2.7 or later and AirPort Admin Utility software version 3.1.1 or later to administer an AirPort Extreme Base Station.
Wireless printing over USB requires Mac OS X v10.2.7 or later.
This AirPort Extreme Card is not compatible with the Mac Pro or the Power Mac G5 Dual and Power Mac G5 Quad computers introduced in October 2005. To add an AirPort Extreme card to one of these systems, please contact an Apple Retail Store or other Authorized Apple Service Provider.
If it's not the dual core it should work - in which case I have no idea what the problem is. Could be a dead card...
Or the card is fine and it's installed in a late-2005 PowerMac G5, which is incapable of recognizing it:
http://docs.info.apple.com/article.html?artnum=302721
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Power Mac G5 (Late 2005) computers will not recognize an AirPort Extreme card if you attempt to install one.