I am eager to find out how your problem is going. I recently bought one of the new 23" High Definition Apple Cinema Screen Displays, and have it connected to my 15" G4 Powerbook which I bought in January.
I've discovered the same strange problem with my screen. Randomly, when ever my screen displays an area which is 'black', I can sudddenly detect countless pixels which flash on and off! The sparkle phenomena.
This occurs some times when I am in Adobe Photoshop in Full Screen Mode. Or even inside a psd file which has a large area of black. Also, when I play some quicktime files, it the black areas. And even when I play a video game, in the black part of the monitor not displaying game vision.
I wondered if this had anything to do with my laptop's video card. So I asked a friend who had also recently bought the new 23" High Definition Apple Cinema Screen to run with his G4 powerbook if he had experienced anything similar. His answer was no, his monitor was working perfectly.
We decided to connect his G4 Laptop to my new 23" High Definition Apple Cinema Screen and see if the problem persisted. Sure enough, their was the flashing pixels in the black areas!
Makes me think the problem is with my monitor.