I have this problem which has been happening for about a month now. My iphone 4 will be happily on wifi and then just stops and reverts to 3G only. Going into settings etc and turning wifi off and on etc won't help, it is as if it can't see any wifi at all. If you reboot the phone it will straight away connect and off you go. But then it drops again and won't for the life of it reconnect. Same for all wifi connections, not just for home or work.. eg at an airport won't find a thing but then you reboot and away it goes..
Same for reset network settings.... Is this just me?
My next thought is to back up the phone and restore a complete factory reset to bare bones and try that...???
I am also experiencing this. It started after my old iPhone 4 was replaced cos the power button didn't work. I got the "new" phone and the new software update and have experienced this issue ever since.