possible cause :
oily traces on screen's backlighting diffuse system ( I do not know the english word/expression ) or some plastic support behind lcd was deformed by heat form a semiconductor IC or a smd too high so it touch the backlight system.
even a small deflection can cause differences of light.
after my n900 honeymoon i will disasemble it to investigate this.

i have same issue with my old FSC T830 after i wash screen with Isopropyl alcohol and it infiltrated behind lcd.
even I drying after complete pda and display dissasemble i don't get perfect clear screen.