Don't mean to hijack the thread, but I have pretty much the same problem.
It's surprisingly difficult to find information on X10. Everything is about these massive home-automation systems or interfacing with a computer.
I have a detached garage with an outside light. The switch is inside the garage.
I have a two-gang switchbox at the back door to the house that controls the porch and driveway lights.
All I want to do is replace the two gang box with a three gang box, and get some sort of switch in there that will control the garage light without digging up the yard to add an additional wire to the garage and tearing up the wall to run a switch. X10 seems like the solution, but darned if I can find some solid information on how to do it.
I guess what complicates things is I don't want a dimmer, I don't want to control it with my computer or a key fob or some George Jetson panel and I don't want my 1920's house to have a silly little square or a decora fixture.
Why can't I just get a simple standard toggle-type switch that looks like all the other toggle switches in my house?
Sorry for the rant. My opinion from my research is the X10 people have complicated themselves out of an entire market. There's a whole neighborhood of old homes around me that could use a simple solution for the situation I have without wanting the huge complication that all the vendors seem to be pushing.