I too am having the same problem as many others.
My iPad 2 stopped connecting to any Wifi network immediately after updating to 6.1.3
My iphone 4s on 6.1.2 works fine, have not updated it yet due to this. All other Apple and non apple devices are connecting to the wifi router with no issues.
I can have the ipad directly next to the router and it still will not see it. I have tried
So far I have tried the following solutions i have found:
Set freq to 20MHZ only.
Set channel to 6 , no change, set to auto no change, set to 11 no change
Restored ipad through itunes both normal restore as well as DFU mode restore
Reset router to factory settings and chose new ssid and passphrase - also tried with no password.
Have tried just about every other thing I can think of.
Starting to get fed up.