Brian has given you a TON of useful information.
I had looked into that whole-house RF receiver unit (I think it is this one:http://www.smarthome.com/31276/INSTEON-X10-RF-Wireless-Sensor-Receiver-EZX10RF/p.aspx
) when I was having such severe problems with my X10 that I was contemplating a full switchover to Insteon. (Turns out all I had to do was buy my neighbor a new CFL bulb!).
From what I remember, this unit can map and X10 address to an Insteon on, which would allow you to use a PalmPAd to turn on or off an Insteon device or scene. Unfortunately, I think that's the opposite of what you need.
I think the computer interfaces, and their associated software can do what you need. Aloternatively, you could get the EZIO40 (or similar), and connect it to either a hacked X10 remote, or an X10-version of the multiple-input unit. (It is like a few PowerFlash modules rolled into one - I don't remember the name of it though). You could then wire the contacts up, so the Insteon command would trigger the X10 command on the other unit. Seems pretty Rube-Goldberg-ish, but it would probably work (I've often considered solutions which would have me wiring the output of a Universal Module to the input of a PowerFlash. Seems silly to me, but sometimes it is the easiest option.
Another way to go might be to use the Insteon equivalent of the X10 TW523/PSC05, along with an actual TW523/PSC05 and an Arduino, and write a "translator" app. I'm getting ready to do something similar to *half* of that (monitor my X10 system using an arduino hooked to the digital port of my XTB-IIR - which emulates a TW523).
It could listen for specific Insteon commands, and then transmit the appropriate X10 command. You could even make it go two-ways, and have X10 commands that trigger Insteon ones.
From what I've read (I haven't gotten time to play with it yet), the Arduino code is pretty simple - there are ton of examples for interfacing with X10 out there already. I have to imagine interfacing with the Insteon side can't be that much harder (I saw some examples, but didn't look into it since it didn't apply to my project). I know the Arduino hardware is cheap, and I've seen TW523/PSC05 units online for pretty cheap. I don't know what the Insteon module will cost you.