Dan,
The RF signal used by X-10 devices will not be transmitted by the Powermids. If you want to control X-10 via your Pronto you do need some sort of IR->X-10 converter, be it the Ocelot, JDS, or the inexpensive IR543. Whatever one you get, you can use the Powermids to carry your IR signal to the IR device from any location in your home, provided you have a Powermid Transmitter in each room you want to use it from. (you can have many transmitters going to one reciever near the IR->X-10 device) Hope this helps!
-erk