A number of iPhone 3G purchasers have reported yellow-tinted displays. Apple reportedly claims that the tint is a result of a warmer display that will generally produce more accurate colors and deeper blacks.
Richard Baguley conducted some tests on an iPhone 3G and a first-generation ipHone using a CS-200 chroma meter and Erica Sadun's Light application. The color temperature was between 6800-7054k on the iPhone 3G vs. 8294k on the 2G. At maximum brightness, the iPhone 3G is noticeably brighter.
It appears that cases may be exacerbating, if not causing, this issue for some users.
"As many have reported, I've too noticed a yellow tinge on the screen. What I found, however, was that this seemed to be due, in part, to the case I was using, which was blocking the new light sensor. This caused the iPhone to think it was constantly dark. Using a hole punch, I punched two holes to the left of the speaker hole in my case (use a flashlight to find the exact positions). This fixed both my light sensor and my proximity sensor (so that the screen would shut off when using it for calls). One side effect to this was that the yellow tinge seems to have gone completely away. I suspect this might have something to do with the backlight being somewhat tinted when on its lowest settings."