Not sure how you want to control this device with X10. The Intellifire controller is a fairly sophisticated device. Page 43 of the following link shows the installation. http://content.hearthnhome.com/downloads/installManuals/4047-132.pdf
The controller offers (page 47):
- Flame height adjustment
- Fan speed adjustment
- Automatic temperature control (built in thermostat)
The interface to the device is not specified - being a 3 wire interface they must be using a form of serial data (not sure if it's ac or dc).
There is a manual over ride rocker for battery powered operation (when the AC power is off). I suppose you could use a X10 contact switch in this position. When operating on battery power the controller is disabled.
As I said, it depends on what you're trying to accomplish. I don't think I'd feel comfortable allowing an X10 device complete control over a fireplace (at least not without manual lockouts).