Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple Have Fixed iOS 6 WiFi Problem
Earlier today we reported that iOS 6 users were reporting WiFi problems but it now appears that Apple has resolved the issue.
The problem seemed to be a problem with the way Safari handles URL redirection on Apple’s side of things and Apple are saying they have now fixed the problem. There was no fix possible that any of the public could do it was down to Apple because it was a problem with Apple’s “Captive network detection page” 404ing.
If you are still having a problem with your WiFi on your iOS 6 device try turning your WiFi on and off in settings and then tap on your wireless networks little blue arrow and click “Renew Lease”