The majority of 3GS buyers i.e. those upgrading from the 2G will definitely notice the switch from cool blue to warm colours (my brother did without me saying anything) and those who had good 3G screens will also notice.
I wasn't being entirely serious.

However, it's been known for ages that the 2G screen was a cooler colour balance than the 3G and now the 3GS. That doesn't mean either screen is faulty, they're just different. As for the 3G/3GS comparisons, my 3GS is slightly bluer than my wife's (end-2008) 3G, whereas it should be the opposite according to most of the posts on here. As for my 3GS, if I really stare at the screen I can perhaps convince myself that it's slightly warmer at the bottom than the top, but to be honest it's pretty hard to see, I'd never noticed it until I looked for it after reading threads on here and it's only visible at all on a plain white screen, which is not something I stare at a lot.
The real point is most will just accept it as norm. Also a significant amount have been complaining according to the previous genius i saw (which is why they ran out of replacement screens) and he also told me feedback notes are being made on each return/replacement...especially in this first week.
But, based on your own observations of all the screens in the store you looked at, it
is the norm. Also, the reality is that the majority of the million-plus 3GS buyers are not returning their phones: the stores would be 10-deep in people if they were and it would be all over the news. The 3GS screens are just different to the 2G ones, like the 3G ones were, and are likely a bit different to the 3G ones as well.
I've no doubt that there are some faulty screens out there (statistically there are bound to be). The screens in the pics posted near the start of this thread look pretty odd, for example: it's hard to judge though, as even though my monitor is calibrated I have no way of knowing what the poster's camera used as its white balance setting.