Flood lights and Home Automation
There are some productive discussions here:
http://forum.arduino.cc/index.php?topic=9980.0
https://electronics.stackexchange.com/questions/79794/determine-with-arduino-if-220v-ac-power-is-on-or-off

A few ideas come to mind:
1) Stick an photosensor in front of the light connected to your home automation server.  Advantage is that you don't need to modify the light's circuit. Disadvantage is that you need to run 2 low-voltage wires outside.

2) Modify the circuit to have a socket, plug in a small transformer (e.g. cell phone charger) and run that wire back to your circuit.  Advantage is that it is straightforward and uses off-the shelf parts.  This works great if there's a junction box indoors that you can tap.

3) Use an AC-powered relay of the types here: (e.g. Tyco R10)
http://www.industrialelectronics.biz/potter/potter.html
Advantages: safe, disadvantage you still need to modify the circuit.

4) Use a hall effect sensor.  Advantage: no need to modify the circuit, but you still need access to it.

5)  Use a custom circuit consisting of a half-bridge rectifier, a resister network divider, and an optoisolator, which is basically what a wall-wart does, except that it has a transformer in it.  Advantage: cheap.  Disadvantage: dangerous.