Obviously, this is anecdotal, but when my wife upgraded, she had the greyed-out WiFi setting.
I restored her phone from scratch, and it didn't fix it.
We then took her phone to the Apple store, and the tech (I refuse to call them geniuses) said the hardware had failed. Thankfully, she was 8 days (?!!) away from warranty expiration, so she got a shiny new 4S.
I understand that iOS7 did a firmware upgrade, and that can stress the hardware - but no errors appeared during the upgrade process. You'd think an incomplete flash would at least kick out something.