I think there's an error in the schematic on the X10 Tutorial page arduino.cc/en/Tutorial/X10
The pull-up resistor should be on the "zero-crossing" pin, not on the "data" pin.
I tried hooking things up the way it shows in the picture, and the example program didn't work (my lights didn't blink).
Then I thought about it a bit, and tried putting the resistor on the other pin. Voila! Flashing lights!