I am currently on my 3rd 3GS and I have to agree that its looking more and more like it is 'intended' to be the more warmer level; just as most calibrators recommend that we change our HDTVs and Monitors to a 'warmer' colour level than the old fashioned 'white is white' ("cold/normal") we have been used to.
It's easier on our eyes for one (reading for longer periods.. less 'glare' at higher brightness) and even more comfortable in darkness for reading/browsing at night.
Pictures furthermore in comparison do look better on 'warmer' units .. comparing the stock-pictures included with 1st, 2nd & 3rd gen phones demonstrates that the 3GS 'warmer' level displays each picture better.
Once you get over the fact that a webpage that has a white background that you have always been used to being white is now 'warmer' (and warmer than you may have been using on a 3G) and get over the initial 'its too yellow' -- but give it a chance, you'll find that you actually like it more after a weeks use.
Those with 'cold' 3GS units seem to be the faulty ones if the truth be told.
My issue wasn't so much the warmness (my 3G has a warm tone... I like it) but, it went from cold (top) to warm (bottom). It wasn't an even tone. It made things like Mail look weird because the blue would be a different color on the top and bottom but, they're supposed to be the same color.