The only time when I had problem with WIFI was the SSID issue. This when you first connect to an access point let say 'ABC'. Then you decided to protect this access point by giving it a WPA2 key or you change the attributes of this access point. Bang! iPhone can't detect any 'ABC' access point or it won't connect to it.

The steps I did was to restore access point back to normal then iPhone can connect. Then on the iPhone, I press 'Forget Network' so it would disengage. Do the necessary settings on access point and let iPhone rediscover the access point.

Sometimes I have to change the SSID name to something else so it can connect. Then press Forget Network and then change the SSID back to something I want before letting iPhone rediscover. Very sticky problem.

For me, reset all network settings didn't work. It is hooked onto the old SSID. I thought reset all networking settings should allow it to forget everything. Too bad, iPhone don't display a list of SSID or profile it has last connected to. That way we can better manage the connections. Also, with this limitation I wonder how many redundant or unused SSID got stucked/cached on iPhone's OS.

Sometimes I noticed that the iPhone 4 would not detect all the SSID nearby (especially the known ones). It will not list down all. But later it will display the WiFi symbol on top to indicate that it has successfully connected. Weird.