Seems like many of us are experiencing the same problem with our iPhone 4S, the WI FI switch greyed out and not able to switch it on, very annoying isn't it.........
OK, we'll here's how I have just fixed mine (I have to admit that I saw this on 'YouTube' and it worked first time).
Sounds a bit crazy but this has just sorted my iPhone 4S WI FI problem out!
Stick your iPhone in the 'Freezer' for 30 minutes, when you take it out switch it off completely
then turn it back on! My WI FI problem was fixed and now not using up my data allowance.
Try this at your own risk, but it has really sorted my iPhone 64GB problem :) HAPPY DAYS......
let me know how you all get on,
Sent from my iPad