I've just added this to the front page but will add it here too, it does repeat some things I've said earlier but for completeness I'll just leave it all in one piece:
Update
Since starting this thread, I've returned my iPhone and have been lucky enough to get another one and its a lot better. Now, from what I can gather from this thread and people's experiences is that all 3G iPhones have a slight yellow tint compared to the original iPhone. Its not as bad on the whole as my picture indicates.
If you have an original iPhone to compare it with, set both to maximum brightness with auto-dimming off. The iPhone 3G should be brighter than the original one. Yes it will have a yellowish tint compared to the blueish tint of the original iPhone. SOME iPhone 3Gs appear to have a slightly faulty screen/backlight and appear to be dimmer and very yellow compared to the original iPhone. In my opinion, these are faulty.
I'm a lot happier with my replacement phone although I would still prefer the colours a little cooler.
If you have an issue with the yellow tint but its not as bad as I described mine to be then I suggest filing some feedback with Apple about the iPhone using the following form:
http://www.apple.com/feedback/iphone.html
If enough of us ask for it, we MAY be able to get screen colour temperature options and maybe even more advanced colour/contrast/gammut settings etc - something professional photographers would probably like anyway so as to showcase their work on the fly better.
Some people prefer the blue tint, some people prefer the yellow tint. If we ask Apple enough, maybe they will give us some more choice on this. I fully believe this can be far better controlled with a software update. Honestly, there's a good chance that Apple purposefully made the screen appear this way so that the icons on the dashboard and films appear to have far more colour depth and contrast and so would pop out as better to some people.