My wifi is also disconnecting when it goes to sleep, with the iPad I was lucky and took it to the Apple Store in Tokyo and the guy just gave me a new one in exchange, and it had iOS 5.1.1 installed, it hasn't disconnected at any time, sleep or not, I once again get WiFi notifications.
My iPhone 4s is still stuck on iOS 6.0.1 and disconnects when sleep mode kicks in, but I have found a work round for now,:
Go to your settings, WiFi, choose your network, now leave it on that page, turn down the screen brightness to reduce battery consumption and set autolock to never, you will get your notifications as normal.
You will notice that just after " Choose a Network " there is a signal looking for a network every 10 seconds or so, that will keep you connected to wifi, connect to power cord if you worry about Battery.
Or, connect to an internet radio app and turn the sound down, you will still get WiFi notifications, as the radio app stays connected when the iPhone is in sleep mode, could even try the pause button on the app, its Trial and error. 🙂
Not perfect workrounds, and I cannot guarantee they will work 100% of the time, but they both work for me, I am going to sell my iPhone 4s with iOS 6 and try to buy another with iOS 5.1.1 if I can.
Apple are really bugging me, they upgrade a well working iOS 5.1.1 and replace it with something that does not work properly, then they tell us we cannot go back to what works, we have to stick with the new iOS which doesn't work.
Let me know if these work rounds work for you.