All bathroom and kitchen sockets are typically protected (electrical code requires it - anything near water really) with only 1 socket somewhere on the circuit with the test button on it.
I swiped this off of smarthome's info page:
Some GFCI devices don't like X10 signals
Yes, I know, it's hard to believe that something would not like X10 signals. We sometimes get calls and letters from customers who mention control problems on GFCI protected circuits. Typically, X10 devices that are down-stream of some GFCI devices will not react to X10 signals. We have concluded that some GFCI fixtures are attenuating the X10 signals. Some brands have more of a negative effect on the signals than do others.
Is there a work-around solution? Two solutions exist: Find a GFCI outlet that does not attenuate signals. Buy your GFCI devices from a supplier that has a liberal return policy. If the brand you install does not pass X10 signals, exchange it for a different brand. Moving to a different model within the same manufacture's family may not fix the problem. Another alternative is to install an amplified coupler repeater that can "blast" the X10 signals over the lines with a high signal strength. Even after the GFCI outlet, there will be plenty of signal strength at the outlet. Most X10 modules need 100mV for reliable operation.
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Which basically sais some (any) GFCI outlets on a circuit can be sucking up your x10 signal. Plug in a boosterlinc on the circuit closest toward the breaker panel might amplify the signal enough to get past the outlet causing the problem. A switch on the same circuit as a outlet will share the same "powerline noise" while only x10 modules downstream of attenuation problems suffer from signal absorption.
Again switchs and outlets are basically the same source, line load ground neutral.