Believe mdh is referring to "YellowGate" which saw a number of iPhone's hitting retail sales while the glue on the screen was still curing. If memory serves that affected the 3G, 3GS, 4 and 4S iPhones as well as the iPad 2. It was more noticeable on the 4S and iPad 2 because they had the shortest turn around time from production to in the users hand.
*figures that call I took while typing this would allow an answer before I hit send*