I just exchanged my iPhone 4 today because the home button wasn't working well.  First time to exchange any Apple product - and I have had the iPhone since last August.  The "new" one I got was made in week 22!  Home button works fine, but man is that screen yellow.  I hadn't noticed at the store because it had the light bubbles background and they put the brightness way up.  When i got home and set everything, I started to write some emails and noticed right away on the white page.
I don't like it at all.  I am going back on Wednesday.  Hopefully they will let me have another one - which I will check out more fully before I leave the store...