Hi,
we used the following scheme to do that job. Basically the wall switch power a 220V coil of a relay, one contact goes to the Arduino to know the position of the wall switch, the other goes to the lamp via a relay that is on the Arduino it self.
The NC relays is keep open by the Arduino, so the wall switch will never act on the lamp while the Arduino is powered, than the Arduino can drive the lamp using the other relay.
So, the Arduino knows about the wall switch position and drive the lamp. If the Arduino is off or broken, the NC relay will control the lamp directly from the wall switch.
This scheme is used in a home along with the Souliss project, so the lamp is either controlled via Android or Modbus.
Please take care, because not always you can try the 220V twice 
and get help if your local law or your knowledge request it.
Note that the wiring of the GND pin is wrong, you need a pull-down to an input pin.
If you need more, just ask.
Regards,
Dario.
plinio.bmp (68.7 KB)
