I have been communicating with another fellow having problems with his X10 system since installation of a smart meter. It is manufactured by Echelon. In the documentation there are numerous references to LonWorks. Searching that turned up some transceivers for communicating over the powerline. Now here is the interesting factor. The spec sheet on one of those devices quotes carrier frequencies as 132KHz (primary) and 115KHz (secondary).
Since the X10 bandpass is 120KHz, and X10 devices do not incorporate a very selective input stage, it is entirely possible that the 115KHz, and even the 132KHz, will interfere with X10 communication if it is being transmitted during the X10 transmit window. I have yet to find details of the actual communication protocol, so I don't know for sure that the smart meter transmitter would be active during that interval.
Jeff