So I decided to visit my apple store today about this. On display there were 2 white iPhones side by side with different screens. One blue and one warm tinted. I compared it to my own white iPhone and found that I had the warmer screen. All the phones had auto-brightness off and was at maximum brightness. Warmer screen was brighter and the whites looked more white, where as the whites on the blue screen one had a greyer tint to it. Upon comparing the stock wallpapers, I find that the warmer screen had more depth to the colors. You could actually see more color variation in the warmer screen. The bluer tint screen displayed a darker image with darker colors (which made less of a contrast in the picture, the colors weren't as vibrant, and didn't very "pop").
I can confirm the bluer tint screens do exist on the white iPhone 4, and I asked a sales about it. He said that the bluer screens were more of the initial batch of iPhones from last year, and that the panels are shifted into more of a warmer variation now.
Also, I really don't think the warmer screens are "yellow" at all. But is just a matter of preference, and I personally prefer the brighter/warmer colors.