I'm having the WiFi issue too on my 4S. I tried resetting the network settings as someone here suggested and it worked but just for a short time. I guess I'm stuck using 3G exclusively until Apple comes out with a fix.
Try this, I changed it and it worked for me on the iPad and on the iPhone 4 with TELUS.
Go to settings>>Wi-Fi>>then choose your network>> once you selected it and it's connected tap on the blue token on the complete right on the line of your network. Scroll down to bottom of the screen, select Auto on the HTTP Proxy and tap on the renew the lease.
It worked for me, and on the wife phone too. Haven't disconnected yet since then.
Not sure if this solution was posted before or not but this is what worked for me:
I use a Linksys WRT54G Router (old school one)
I have the lastest iOS 6.1.2 on iPhone 4S and iPad 3
I was not able to maintain a constant connection on my wifi network. When phone went to idle/sleep, I would have to connect once again to the wifi network, put my password again etc etc.
I narrowed it down to the fact it was a "wireless security" issue. Not the phone nor the router being an old one. I was using the WPA2-Personal (TKIP+AES). i changed it to WPA2-Personal (AES) and it worked without a hitch. I have been monitoring this for 2-3 days now and everytime I restart my iPhone 4S or even put it to sleep, it connectes to the wifi network just as it should - automatically.