I'm coming to the conclusion that the wall switch itself has gone bad. I have a house full of X10 & Leviton PLC stuff, and everything works fine except this one switch.
The signals are propagating (I can see them in AHP's log and by watching the lights on my coupler/repeater). I've unplugged everything I can on the nearby circuits; noise doesn't seem to be the problem, and neither does there appear to be a signal sucker nearby.
My only problem now (here comes Mr. Murphy, wait for it...) is that this particular wall switch is inaccessible behind the big heavy solid oak wall unit of my bed. (When I moved into this house, I realized the bed would cover up the two wall switches controlling the overhead lights. So I made them both X10 switches so we could control them with bedside mini-timers.) I would need to partially disassemble the bed to get to the switch. Sigh. I may try to get a right-angle screwdriver in there later today, but if that fails, I'll just have to live with it until the next time I need to take apart my bed.