I have had my SGS for about three months. I have had it set on max brightness and also set up so that screen stayed on while on the charger - which is all day when I am at work. This is because I love it and want to see it in all its bright Super AMOLED glory.
Over the weekend I noticed very faint yellowy discolouring in the lower RH corner of the screen where the 'Applications' icon is on the home screen. When looking at screens other than the home screen, there are four very faint yellowy squares that correspond to where the four white squares are in the Applications button of the home screen.
I'm pretty sure this is burn-in so I have activated auto-brightness, shortened the screen time-out and turned off the 'screen on while charging'.
The yellow is not too bad and most people may not notice it - but it's there and its scary.