Obviously the displays differ in ways not described in the tech specs above. Not only that, but the claimed vertical viewing angle for the 20" is no where close to 160 degrees. Roughly 30 degrees above perpindicular to the screen, the contrast is extremely poor. In some cases the contrast disappears or inverts. For example, the gray color used to indicate selection on a window without focus disappears. The shadows around windows appear brighter than the surrounding desktop rather than darker. This has been my biggest disappointment with the iMac. I constantly find myself adjusting the tilt or brightness because of my viewing angle. My iMac is in my kitchen so often I'm standing rather than sitting.
As far as the in-store evaluation goes, most of the in store models are 24-inch. Many shoppers will demo the 24 and then purchase the mid or low-end 20 without evaluating it for such unexpected differences in the display. Seriously, if you wanted the larger HD capacity, would you be paranoid enough to test for differences other than capacity? What Apple has done is equivalent to slipping in a 5400RPM drive without telling you when you pick the 500GB option.
People are acting dumb about this simply because the 20-inch is less expensive. Who cares?? Buyers had the choice between options, along with a description of the two options and their differences. It doesn't matter which option was the more expensive. What matters is that buyers did not have the information needed to make the right choice.