I typically do the same thing (I brought my iPhone back 3 times with-in two weeks until I got the right one) which is why I was trying to gauge peoples mileage with iMac returns. The question was not so much SHOULD I take it back but what was the best method to do so since I had no experience with iMacs.
Thanks,
I've gone to the Apple store (and this is also based on the people working there and their attitude at the time LOL) and asked about something pertaining to Battlefield2142 last year and the first thing the Genius said was "Do you have it with you?"
So I would venture to say, make an appointment, take it with you (yah I hate lugging it around in that box, the top flap is poorly designed unless you retape it) and explain that you are NOT satisfied. Again, within 14 days there's really no questions needed. Otherwise it's within warranty and they should replace the LCD panel.
My only concern with this is ending up with one that's worse than the one you had. Meaning, dead pixels, more noticable gradient, etc. Dead pixels tend to be grouped into "If it doesn't meet the industry standard for replacing your stuck with em" type of mentality.
Keep us informed though on what you do. Lots of people have taken iMacs back with this exact problem, some have had good experiences, some bad. Probably due to the personel but /shrug who knows.
Ohh and another thing to remember, Nice is good, Rude is bad. I yelled at Apple over the phone in a nice way (yah that doesn't make sense but it's what I did) and I ended up with a free iPod Nano (non video).