If your light is controller by your Arduino board, you have yet that information. The tip is simply connect the wall switch to your Arduino (as 3.3V or 5V input) an use it to control a relay that will power on or off your light. Of course, if your board can be accessed via Ethernet or other media, you could even give the ON command from another source.
Have a look to this video, it shows the first release of Souliss framework controlling over Android a couple of lights and an air conditioner. At the top of the buttons there is one feedback indication that is red for OFF and green for ON, while controlling the light it will follow the state coming back from Arduino.http://youtu.be/VuqOFjQTVYg
This release is pretty slow (the actual one has greater performances) but get the feedback directly from the board, without using nothing special, just an Arduino Ethernet and a couple of relays.
The state of the output pin of your board (its values is in your sketch) is the actual state of the light. This will give you a feedback at actuator level, of course if your light bulb is burned, you will not get any advice. If your Arduino is powered OFF you will not be able to power your light, even from the wall switch.
To have your lights in operation also if your Arduino is powered-off, you can use the following schematic, that needs some external components, in this case a relay with coil at 230V and two 12V coils relays for controlling the light.
With this basically your drive the light through Arduino till the NC relay is open, and with the wall switch when the NC is closed. The wall switch position is reported back with an auxiliary dry contact, that became a 5V input for your Arduino.
This solution is used by the developer of the Android application in the previous video, to have full control also in case of failure of the board.