You can not implement it in legal, AppStore-compartible way, without use of APIs which are private on iOS (althought they might be public on OS X).
If you do need basic Wi-Fi scanner functionality, you might use Apple AirPort Utility, just install it from AppStore and turn on Wi-Fi scanner in its preferences.
If you do need some deeper functionalities for in-house testing/investigation, you might want to look into SystemConfiguration framework, I managed to create a test application which reports current Wi-Fi channel the iPhone connected to (but this is definitely for internal use only, it wont be approved in AppStore).