No, you cannot. On the iPhone, turning the Wifi off (or turning Airplane Mode on) effectively shuts your Wifi hardware down, so there's no possible way of doing that on an unjailbroken device.
Apple limits the ways in which you can access the iPhone hardware in order to protect users from fishy apps with root permissions that could potentially drain the battery or put the hardware/driver in an unstable state.
I also don't understand why on earth you would like to do this. If you really need to do this, you can keep Wifi on but disconnected from any Wifi network. Although I suspect that even then you cannot get the list of Wifi networks from an App [1] in an unjailbroken iPhone (Apple doesn't like Wifi sniffing tools either).
If I were a user of your App, I'd like to be certain that when the Wifi is off at the OS level, it's off period. I'd be summarily pissed off if you App accessed the Wifi hardware behind my back.
- Find available wi-fi networks