Until now, 27 iPhone 4 owners reported some yellow areas on display, despite any adjustment of brightness.
It seems that another problem that some users reported refers to a signal error, meaning that if the iPhone is held in hand it has a really bad reception.
Moreover, the iPhone 4 was also tested in order to see how durable it is. Therefore, when it was dropped on the ground, the screen flourished as a frozen winter glass.
On the other hand, this may be only a way to make a bad image against Apple, given that today the iPhone4 started being sold in stores.
Image source: Go4it