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);

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://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1261353753

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 should be added as well:

// Turns on module A1
myHouse.write(A, UNIT_1, 1);
myHouse.write(A, ON, 1);

Error: Sorry, you are not allowed to post messages containing active links to websites or images before you have posted 1 normal messages.

^^ You have got to be kidding me.