I had the same thing. The other option is to put the main X10 RF switch for the lights at the bottom of the stairs, and then, since it's RF remote, you don't need a straight slave switch at the top. Put a 6 or 8 button controller there that not only controls those lights, but other lights in the area. Or, you can create macros with any remaining unused buttons. I used 6 switches on an 8 button controller to cover the entry light over the stairs, the bedroom hall, and the living room. Then I left the bottom two buttons open and programmed them as an ALL ON and ALL OFF buttons.
The 8 button without the integrated dimmer (don't need it if you purchase dimming switches) is $74. Here's the lincs:
[Link: smarthome.com]