How would one go about checking for gradient issue?
Set the desktop background to "Solid Aqua Blue", "Solid Aqua Dark Blue",
or "Solid Gray Medium."
On the 20" ALU, drop some colorful icons at the top and bottom edges of
the screen -- and compare their color saturation and brightness. Or open
an iTunes window with simple list view (no cover flow, no iTunes store),
and check for top-to-bottom (non)uniformity of the horizontal stripes.
On the 24" ALU, compare the brightness of the left and right sides of the
screen. Here are some examples of 20" ALU, 24" ALU, and 20" white:
http://picasaweb.google.com/TheLooby
Also try a solid white or light gray background and look for color uniformity.
Some 24" displays appear "dingy yellow" at the right-center or right side of
the display. (The photos linked above do not exhibit this defect.)
LK