Its a little annoying, but not a big deal. No, its not an optical illusion either. I compared it side by side with my 3g and there is a difference. I have the 924 32gb black. I didn't even notice it before now, and I'm a graphic designer.
My biggest annoyance is the brightness level compared to my 3g, although that is adjustable. I wonder if this is where Apple is getting the improved battery figures? Using the same exact display settings for brightness, tested in different lighting conditions at different times of the day, my 3gs is nearly twice as dark as the 3g. I have to raise the brightness slider up much further to match to two together. If I turn off auto-brightness, however, they are nearly the same. So I credit it to over-zealous power saving. Its annoying though because the baseline for my screen is too dark. I turned it up today--hopefully it doesn't hit the battery life too hard because in some brighter lighting conditions it may be brighter than normal? Not sure.
Edit add:
Looking at the 3g and 3gs side by side, from varying angles, the 3gs had much better viewing angles. I couldn't get my display to turn orange at angles like my old 3g would. The displays were set to the same brightness and the 3gs was brighter from all angles.
The colors are also a bit cooler than my 3g but still warmer than my first gen. Aside from the yellowing bottom, the white balance is really good and photos side by side show a bit more detail and clarity near shadow areas on my 3gs.
Either way it looks away better than the TN piece of crap on my wife's Dell. Although it doesn't come close to the quality of my H-IPS calibrated monitor either. The yellowing only slightly annoys me now when in landscape in Safari on a mostly white or new page. I wish I hadn't read this post! However, I will most likely forget about it in the coming weeks.