This might not sound like something you want to hear, but keeping taking them back till you get a good one. If you're going to drop hard earned cash you should get a machine without dead pixels.
As long as you don't exceed the 14 day return policy (total refund) what are they going to tell you? They don't have much of a choice.
They can't sell the ones you returned as new anymore and that alone will be incentive for them to fix their issues with QC.
I made 9 round trips to the Apple store till they got it right with my MBP (2 screens, 4 logic boards, 2 Airport cards and 1 whole display assembly). They gave me a brand new one after that. Hopefully when I switch to an iMac here in a few weeks I won't have to go through the same ordeal again!
Apple has a lot in common with the V.A. (Veteran's Administration), if you're persistent enough they'll eventually make things right.