It isn't going to happen to me because when I'm on wifi I turn cellular data off just to be sure.
This whole thing could be a big problem. Imagine how many kids have iPhones, grandma and grandpa, etc. I could see data plans being totally abused.
I had to turn off cellular when I was on wifi with verizon b.c it was eating up my data. Their response was, "that is normal"..uh no the hell it is not