My 4S also was unable to detect wifi networks. I heated my phone with a hairdryer and the wifi settings open and at a certain point networks were detected. I thought I was fixed but the next morning wifi was gone again. So I did the same thing but this time after networks were detected I did a hard reset (power and home buttons together) and this seems to have done the trick. I think that whatever heat does to reset the wifi chip, it's probably overkill and possibly damaging to the phone to heat it beyond this point (i.e. till it shuts down). Likewise, the rapid expansion and contraction caused by heating it and putting it in the freezer seems likely to cause connection problems inside the phone, due to damaging solder joints.