After a few hours of frustration, I finally got it working (It wasn't clear from most of the documentation that I needed a 10k resistor between the Zero Crossing line and +5v).
The X10 Protocol document shows what is required to interface to the TW523/PSC05. That is available here:
http://jvde.us/info/X10_protocol.pdf
Don't ignore the clamping diodes like one manufacturer did if you want a robust interface.
There is also an extended code document (Google xtdcode.pdf or xtc798.doc). The most common extended code is 62 bits long. The CM15A transmits it as a doublet, which will be repeated by the XTB-IIR.
Note that the real TW523/PSC05 will not receive extended codes, but the XTB-IIR will.
Jeff
Jeff -
Thanks for your reply. I had already looked at the protocol documents, but I don't really understand them well enough yet. I took another look today, and I'm only slightly less lost.
I'd really love to find some examples of using Extended Dim commands with the library I'm using, but I haven't found any yet (While Google is my friend, it has been more like the friend who has to tell you the entire backstory on the history of restaurants, before telling you where they went for lunch yesterday).

I did find a second library here:
http://code.google.com/p/arduino-x10/downloads/list , but that one looks a lot more complicated (although it can do a lot more, too). I had tried it last night, but only BEFORE I fixed my missing resistor issue. By the time I was able to send simple ON and OFF commands, I was too tired to go back and try the other library. That's on my list for this weekend.
I'm trying to decide if want to have my current Arduino (the HVAC monitor) talk directly to the XTB-IIR, or have it talk to a second Arduino (by setting a pin high or low - using an opto-isolator would be smart), and then have the second one send the appropriate commands to the XTB-IIR. The advantages of using a second Arduino are: (1) I don't need to use as many pins on my already crowded HVAC monitor, and (2) I can add other things to the second one later on (like the "flag status" LED panel I've wanted to build for a while). I would almost be using the second Arduino like a really fancy programmable PowerFlash in that case (and if I were to add an Ethernet shield to it, now THAT would be a really cool project - I have to add that to my list!).
A third option would be to move the whole project (HVAC *and* X10) over to a Mega (I have one - that's what I've been using to build a "first draft" of anything I try), which would give me plenty of pins to use, as well as allow me to have enough extra pins to add in the Ethernet shield (or buy one these - which look REALLY cool! :
http://www.freetronics.com/products/ethermega-arduino-mega-2560-compatible-with-onboard-ethernet ), and then rebuild my HVAC monitor shield on a Mega-compatible prototyping shield. I'd love to add the Ethernet capabilities at some point down the road (so the Arduino would post the data to the MySQL database via a PHP page on the server), but that's a long way off at this point.
Have any of the forum members (or non-member guests who want to become members to reply) played around with interfacing the Arduino and X10 via the TW523/PSC01/XTB-IIR, and would be willing to give me some code examples to work with?