According to the company, the Xperia S is transient and temperature-dependent, therefore the yellowish patches only shows up when the device gets sufficiently warm. Sony claims that this issue should have only affected a small number of handsets and that they have since modified its production process to avoid the same issue from cropping up in future Xperia S handsets.
It would have been great if Sony had caught this before the handsets rolled out, after all, the Xperia S is the first Android smartphone the company has launched since it dropped the Ericsson moniker and we’re sure that they would have loved to make a great first impression. In any case if you’re an Xperia S owner experiencing the problem mentioned above, you should probably contact Sony to see what can be done about it.