I have the exact same issue.
Iphone 4S working 100% for just over a year (new phone) until ios7, soon as that loaded my wifi started acting up and eventually would not turn on.
I went into the "genius bar", I was kind of disgusted that they simply plugged the phone in, and told me that my wifi chip had failed, I am out of warrenty and need to pay $300.00 to have it repaired.
I argued that its very coincidental that it works perfectly until ios7 upgrades and then these issues start, I was told there is no issues related to iPhone 4S and ios7.
When I said, thats not what I see across all the forum, this was ignored.
So I went and pulled down the ios7.02 ipws file from apple, since (thank you Apple) I cannot downgrade anymore.
I put the phone into recovery mode and pushed a fresh install of 7.02, I noticed magically wifi starts working while im activating my phone, I activate everything to see if I can get it to act up again, so far so good.
Then I get a popup saying there is a carrier settings update, I click apply, suddenyl my wifi falls over again, yet again I am unable to get it fixed. (back to square one)
So I start looking around online for any topics related to ios7, carrier update settings and wifi, no luck here.
I once again put the phone into recovery mode and force down ios7, the phone starts and whats activation and wifi yet again is working.
This time I ignore the carrier update popup and all is well for a few hours.
I go into Settings, Carrier and look at the carrier settings in here, I confirm these are already set correctly to those of t-mobile, I try to manually re-enter them and apply to see if it will stop the popup, it didnt.
So after a few hours of perfect wifi, I go into Settings>General>About and get the popup again (it always appears if you go in here if you didnt accept it), I accept it and then look at the carrier settings, it does not appear like it changed anything.
I have turned the wifi off and on a number of times, performed a number or restarts and and used the phone now for another few hours, so far wifi is working perfectly without issue.
I'm not sure if the carrrier update did or did not have anything to do with it or if it was just coincidence.
Thought Id post my observations so far, maybe it helps someone figure something out.