Hmmm, are those G4 powerbooks the same? The brightness is going like this: 12<15<17
Also check this forum for a thread that was about cleaning the screen. I don't think that is causing the huge difference in brightness, but a clean screen is always a great thing. In worst case I would visit your next Apple Store and compare to their screens. Maybe Apple secretly updated the screens?
iBook 600; 12''; 640mb; 8mb Rage; DVD-CDRW-Combo, 20GB
P4 1.6; 2x80GB Raid1 (file-server)
tiBook 1Ghz, Superdrive, 768MB, 64mb 9000, 60GB