I can't speak for all X-10, but here's a little info on what I do know.
1) X-10 developed their own communication protocol at least as early as 1982 (that's when I first saw it). It's been around a while.
2) The original X-10 modulated the signal onto the power line (modem). Some newer modules are now wireless. This is why the base controller has *both* an antenna and outlet interface.
3) Early problems were observed when the signal traveled too far down the power line and triggered devices in other houses. I believe this one is mostly solved.
4) Sometimes when a good surge-suppressor with EMI/RF protection is plugged in, the signal gets filtered and the modules don't work. This can be tested by temporarily removing any surge strips with this protection.
5) Developer modules are available for designers who want to interface the outlet with microcontrollers that can speak the X-10 protocol. See
Application Note for details and
Parallax website for basic information.
6) X-10 is a communications protocol, so you can use X-10 modules from other companies like Tandy (Radio Shack).