Mike13, If it is a cable box you are working with, simply get a 9VDC regulated adapter and connect to one of the Power Sense ports on the RFX9600. Then, plug this adapter into the extra power jack on the back of your cable box. This solution is "real" cheap and will work just as good as any sensor.

My understanding is that the RFX-9600 Power Sense circuitry is voltage-triggered in that any voltage above 3-5 volts will indicate a "Power On" status.

EDIT - I must assume from avtech's post below that the user want's credit for the idea. Well then, see the the link below as she was the one who first mentioned this.