It sounds like there is a slight dip or imperfection on the back side of the digitizer. It is collecting more light from the backlight. If it is under warranty take it in for repair. They may exchange it so I would back up your information on it. Digitizers are not made to have above gentle touch pressure. The touch screen is glued directly to the LCD I would imagine you would get a kind of purple color pressing firmly on it, since that is the native color of an inactive LCD. If it is not under warranty and not interfering with reasonable normal use I would live with it. If you can't do that you will have to replace the screen assembly. The instructions for replacing your screen are here. New screen assemblies are here.

I have it too... Monds a go I replaced a front iPhone 4 and it seems like there was a white spot in the LCD. I replaced another and iPhone was working fine. I tested the LCD in another iPhone and the white spot was gone. Now Monds later I replaced the battery from that iPhone and now the white spot is back... Could it be there I pressure from the back? I know the LCD works fine but somehow there is something that's gives the white spot... Anybody?