Limitations of Apple DVI to ADC Adaptor
The bottom line, after three long calls to Apple Support and a second DVI to ADC Adaptor, is that the DVI to Adaptor WILL ONLY SUPPLY FUNCTIONALITY to the Cinema Display buttons on video cards that have two different ports - DVI and ADC. If you don't care about the button function on the Cinema Display, then you'll be satisfied with the DVI to ADC Adaptor. All though it is expensive at $99.
At Apple's suggestion, I took my new G5 to a local Apple Certified Repair Center. A tech type worked with me and we tested the ATI Radeon 9650 video card and the DVI to ADC Adaptor on one of their Cinema Displays. It did not work. (That is it provided video signal, power and USB, but the buttons on the display did not work.) But..., when the tech popped in a ATI 9800 Pro card with a DVI port and a ADC port, the DVI to ADC Adaptor worked on the DVI port - it provided electric power, video signal, USB and Cinema Display BUTTON CONTROL.
You can look at a card, with out installing it, and determine if the DVI to ADC Adaptor will work in all respects. The totally functional cards have an extra connection tab that goes into the slot. This supports the Cinema Display Buttons!!! Also, these cards will have an ADC port.
BEWARE! Apple support techs DO NOT UNDERSTAND THIS. Their documentation leads them to believe that the DVI to ADC Adaptor will work with the ATI Radeon 9600/9650 cards to provide Cinema Display button functionality. Wrong, Wrong, Wrong!
Therefore, the real purpose of the DVI to ADC Adaptor is to allow you to attach a second ADC Cinema Display to a video card that has a DVI port and an ADC port.
Apple support techs have (to a person) tried to be helpful. It was their idea to have me work with a local Apple Certified Repair Center to determine what was happening in my case. I've shared with them what I learned. Hopefully, they'll now be able to be more targeted in their support regarding this issue.