As Brian noted in his iPhone 3G review, its screen is warmer (more yellow) than the previous iPhone's. That's intentional, Apple's senior director of iPhone product marketing, Bob Borchers, told MacWorld in its first official response on the matter. It's meant to make the display "more natural" and less like harsh fluorescent lightning.
Wireless Info has the exhaustive numbers on the matter. The iPhone 3G's screen is not only brighter, its color temperature is around 6800 to 7054 Kelvins, while the first iPhone's was about 8300K. So it is definitely more yellow. (Here are some head-to-head shots.)
That said, when we had two of our iPhone 3Gs side-by-side, one of them was noticeably yellower than the other, even though all the settings were the same. So there could still be some issues out there. Let us know what you see. Update: Ars confirms a fix we saw on Macrumor's forums-if your screen is too yellow, check your firmware and update it to 5A347 if you've got 5A345. That seems to reduce the pee tinge. [MacWorld, Wireless Info]