X10 has unit codes in addition to house codes allowing for many devices to work.http://www.x10.com/support/basicx10.htm
I have used a ton of X10 but only install it at my own home for fun stuff or integration demos. If you only need to automate a few lights in a fairly small area, then X10 may work for you. Do not attempt to build a product or service around X10. X10 can be tough to implement whole house due to it just plain not working in some parts of a house (i.e. 1/2 the house). Also, noise in the power lines can turn on/off your devices in some cases. There are devices you can get to get around some of these issues, but they cost money and next thing you know, you have spent actual money on X10.
The fixtures are usually very low quality and some have issues with the new compact florescent lights, I just removed my X10 switches for that reason a month ago.
That said, if X10 is a good fit then, here are some tips:
Using something like arduino and the device you reference to control the signals is a good idea. Using a wide open RF plug in module that allows anyone to control your lights is on the verge of cheesy.
The best way I have found to implement X10 (not related to Arduino) is to use the same type serial computer interface module to control it via a computer. I then created a web server run on a thin client in each room would then read the web command and fire off the X10 command for that zone. Using a controller for each room helped assure the power line interface signal could make the entire zone.
How I would suggest implementing something similar with arduino would be to use the Ethernet Shield with SD card reader. This would receive the web command and control the lights for that zone and use the code in the tutorial you referenced . This web server could "abstract" the light codes from the device name. So if your codes changes, you update the data on the SD card. So when you say ' LR1=OFF, LR2=ON' .. it converts to the correct house and unit codes. How far your X10 travels in your house will control how many zones you need. The Ethernet shield gives you whole house capabilities.
Best of luck