(I'm a designer, and amateur photographer) In the first week of the iPhone 5 release I noticed that my screen was slightly yellow. But not as yellow as my brother's. I compared my phone to the various display models in three UK Apple Stores and they seemed whiter, with better contrast, deeper blacks and with a higher brightness level, with phones set on maximum brightness.
A collegue at work also had an iPhone 5 that had better white and contrast.
It is important to note that the iPhone 5 does *inherently have a warmer tone* compared to iPhones of old, but some phones are overly warm as reported by some users here. My seemed to be in the middle ground.
After several weeks of ownership of the same phone, I returned to an Apple Store for a re-evaluation, and compared mine to the display models again. Surprisingly my phone is no longer that yellow.
So two scenarios might have played out.
1. My iPhone has become less yellow, which I think it has (although I am aware of the possibility that you can become 'used to' the display) hence my desire to benchmark again with the display models.
2. Apple have started displaying more yellow representative phones rather than the previous white phones, so they can manage expectations and reduce complaints/returns of yellow phones.
For users of an iPhone within acceptable limits of yellowness (the inherently warmer display) I think it is important not to *just focus on the white*.
While in store, I compared the same photographic images on my iPhone, and Mac Book Pro std, Mac Book Pro Retina, 13 and 15 inch, Apple Display, iMac and at home compared to iPad Retina and iPad std, and generally Apple have gone warmer or warmer than the iPhone 5. None of the devices offered higher contrast or brightness levels than the smaller iPhone 5 screen. The Retina laptops were particularly disappointing against my iPhone 5, images were totally flat, and contrast of text low. I used sample images from www.dpreview.com
I also compared my iPhone 5 to my wifes iPhone 4S which had a cleaner white,- and photographic images lacked contrast, depth and colour range compared to the warmer iPhone 5. Yes, whites compared to the 4S aren't as 'clean' but in the grand scheme of things and compared to other devices it definetly appears superior.
I've also compared it to my work Thunderbolt 27 inch display, and the iPhone 5 has better brightness and is less yellow. The Thunderbolt display which I haven't calibrated is quite 'yellow' out of the box compared to iPhone 5. I've adjusted the Thunderbold display using the standard controls and it's still more yellow than the iPhone 5 and Mac Book Pro 15 inch std I have. I've even compared it to my older iPhone 3GS, which is really blue, too much so, and the photographic images are lack lustre compared to the iPhone 5.
So as I said in the grand scheme of things, the naturally wamer iPhone 5 screen for photographic images isn't bad at all. There are variances of warmness, and obviously there is an acceptability level for individuals and for Apple.