[Simon]

I'm not quite sure if this is common knowledge here, but I just discovered this today. If you have an Apple Cinema Display (the new Al models) you need to attach the screen's USB cable to the Mac if you want to be able to set the behavior of the screen's buttons in System Preferences.

This morning, I had my 20" ACD attached to my PowerBook as always, but the screen's USB cable wasn't connected to the PowerBook. I noticed that in System Preferences there was no display options tab. At first I though this was some bug, but then I remembered that I once had an Apple 17" CRT Display that required the screen's ADB cable to be connected to set the display's prefs. So I reconnected the screen's USB cable to the PowerBook and restarted System Preferences. Voilà, display options back.

However, why is USB required to set the behavior of the buttons? It's not as if their functionality would be controlled over USB (the controls work even with a disconnected USB cable, so I figure the signals go over DVI, not USB). Anybody have an idea, why the USB cable has to be connected?

[Cadaver]

Because the functionality of the buttons IS programmed over the USB bus. There is no data channel on the DVI bus to allow for programability. The ADC monitors didn't need a separate USB connection because the USB bus, along with DVI and power, was carried over the ADC connection.