That's funny. When I took mine back I mentioned it to a few floor associates on the floor and they all gave me this bewildered look. I think they know or heard of the yellowing but are instructed to play dumb. Any acknowledgement from any Apple employee will be an admission to the problem and will make Apple look bad if don't take measures to resolve it across the board.
If you can't easily reproduce it or the Geniuses claim they can't see it then I recommend loading up the gray image I posted
here. The white rectangles at the top and bottom should look different when looked at dead center. When I took mine back the Genius couldn't see it on a white webpage, but was convinced of the issue when he saw the gray image.
I hope it helps and good luck!
Edit: I'm also waiting for them to receive more in stock.