RFID Modules would be a great addition to the X10 Line up...
X10 could make a standard looking module that has the house and unit code as normal, but instead of receiving on that code, it would send that code when an RFID Tag was within range of it, and send an off code when the tag left the area, like the motion sensors act in that sense...
and to get fancy, they could have the RFID tags be one for each house code, or just in sets of 4 like the cameras that way AHP users could take things further and have certain events happen for certain tags.