The circuit diagram on the X10 tutorial is incorrect. It does not work at all.http://www.arduino.cc/en/Tutorial/X10
This has been discussed before:http://arduino.cc/forum/index.php/topic,17333.0.html
The problems are as follows:
1) The Zero Crossing and Data lines are swapped.
2) The jack orientation is confusing and should be marked with pin numbers.
Here is a suggested replacement:http://imagepaste.nullnetwork.net/viewimage.php?id=1689
Also, the only X10 command example shown in the tutorial is the "ALL_LIGHTS_ON" command, which not all modules respond to. Addressing a single module is not particularly intuitive. An example of this could be added as well:
// Turns on module A1
myHouse.write(A, UNIT_1, 1);
myHouse.write(A, ON, 1);