I'm no expert, but that last screen looks pretty good to me. As others have said, there's viewing angles issues with these IPS screens, and when tilting your head the slightly warmer tint in the bottom of the screen goes away. I also have some slight backlight bleed in the corners when cranking up the brightness, but it's mostly a viewing angle issue. I did not get image rentention, scratches, dead pixels etc. I would not risk getting a new unit with one of these issues, just because you want the illusive "perfect" iMac screen.
Of course, some people here have serious issues with their iMacs which should be addressed. I'm just saying some here obsess over minute details, and that "perfect" screen is pretty hard to come by, if possible at all.
Maybe if people that claim they have an iMac 27'' with 0% backlight bleeding could post a picture of their screen at 100% brightness? I doubt we'll see it...