The higher numbered weeks (40's-50's) from late last year were severely plagued with screen flickering and yellow tinted screens. Apple reps have been stating that the 2010 iMacs (weeks 1-5 on) were supposed to be fixed and free of screen defects. This has shown not to be the case. That is why they are complaining. People are vey upset that this problem was widespread beofre 2010 and Apple claimed that they fixed it. So, here you have a lot of people comparing notes on their iMacs from these recent weeks. If you comb through the threads, people are complaining about theses weeks (1-5 on) and the earlier ones from late last year.
Right now, it seems like the likelihood of getting a bad screen are high. Perosnally, I would wait until Apple decides to fix the screens.
Here's some info:
http://gizmodo.com/5459384/the-faulty-imac-saga-chapter-3-we-have-your-internal-memo-apple
Apparently, Apple is telling customers that its normal and NOT to REPAIR or REPLACE the iMac. So, if you get a bad one, They will not offer to repair or replace it right now. They tell their CSRs to explain to the customer that it is normal to have some discoloration.
"Buyer beware," as the saying goes.