I'm currently in the middle of the same issue, as it were. Got my iPhone 4 32GB just before Christmas and made the 'mistake' of comparing the screen to my outgoing 3GS. Although the iPhone 4 had deeper blacks the yellow tint was rather annoying to me.
Went to my local Apple store in Kingston-Upon-Thames, London and compared with two display models, two personal geniuses iPhones and finally the managers, all were how I wanted mine, a cool white tone.
So after swapping twice and going through a handful of refurbished models I phoned the online store and detailed my problem and they are now sending me a brand new one, if this turns out to be yellow then I might just hold onto my trusty 3GS until iPhone 5 hits, with hopefully all the niggles sorted out.