I am a bit concerned with the way everyone is jumping on the OP here and making out like he is being foolish or has too high expectations or something. Since I have the exact same 23" ACD at home, and since I just got a 20" iMac at work yesterday, I think I am more than qualified to comment that that the screen (on the iMac) actually is a fairly "crappy" part.
As background, I am a Mac computer techie at a major University where I live and have years of experience with Apple monitors, screens, laptops etc. The new screen on the 20" iMac *is* most definitely the worst I have seen.
In addition, the computer that I replaced with the new 20" iMac was the previous generation 20" PPC G5 iMac. So for those of you thinking he is wrong to compare the new iMac to the 23" ACD, I can confirm that the new iMac 20" screen is fairly awful even compared to the older iMac screen. I have a suspicion however that it's not so much the part itself, but the glass in front of it and the way it's lit as people are noting similar experiences on the new iPod Touch.
With the older type of (non glass-covered) screens, you can move around in your chair or look at it from many different angles and the colours and lighting remain essentially the same. You can rely on the fact that what you see is actually the colours that are there. Whereas with the new screens, you need some kind of vise to keep your head straight. If you sit directly in front of the new iMac, and tilt the thing exactly so, and adjust the profiles "just so" (about 10 extra points of gamma does it for me), then yes, you will see the correct colours and shades (at least in the very centre of the screen). Any other angle of view, and you will see a variety of darker/lighter saturated/less saturated colours. Even then, if you look up at the top of the screen everything is darker and over-saturated, if you look at the dock, the icons are washed out and under-saturated.
You have to bob your head around all over the place jsut to see the screen properly. I actually got a neck ache yesterday twisting myself around in front of the new iMac trying to see the screen.
This is far from a huge tragedy and considering the price of the computer, nothing to really complain about for the average user, but to me there is no argument that this is most definitely the lowest quality Apple screen ever. This screen is (effectively) hardly any better than what we give to our lowest end users which is a Mac mini attached to some bulk Ben-Q VGA crap. If this was a product I bought with my own money from the store I would be taking it back already.
Self Ranked #1 in Apple Display hyperbole!