I am experiencing a return to full usage, but uncertain as to why.
I retrieved my second monitor as well. It is hooked up to the G5 and seems to be functioning perfectly. At first, both monitors seemed to not want to work. I hooked up the power supplies and all was well. Then the main DVI video cable- without the USB or the Firewire . For the Mac Pro- the small light would not come on at first. I tried hooking up the USB and Firewire cables and still nothing. I unhooked everything and shut down completely. After rebooting I hooked up just the DVI cable again. Eureka. The small light in the lower right came on and the monitor came to life. I went into display panel in preferences and chaged the power button to the first choice in Snow Leopard - powers the monitor on and off.
Then I turned to the G5 and did just about the same thing. The monitor is now up and running and functioning well.
In both cases, the displays seem to just decide to cooperate... Go figure.
Hope you find success. I am advised by a technician friend that the Apple 30" displays can be repairied and or restored to full functioning for far less than the new 27" monitors cost. The massive screen work great for Final Cut Pro 7 and I adore them. So glad I tinkered and loved them... I have no idea if an internal board needed an extra bump or what.
best Wishes - Tim