To be honest, I thought I saw these yellow splotches (small splotches) myself, early when I bought it.
I thought I was looking at a bright yellow light (like glancing at the outdoor light) and then looked at the iPhone screen. (And you know when you stare at something bright and look elsewhere quickly, you get that natural aversion to bright light affect.)
Then the next day I saw it again on the iPhone, knowing that I didn't look outside at all.
Thus, you may be right, it might have something to do with the heat generated by the phone.
However, since then I've never seen those yellow splotches again. I have had the phone since the iDay.