Hi Phil.
Well I'm not building one, just replacing it 
I've used the CM15PRO for a couple of years and was annoyed by the short RF range and lack of multi platform support. I've also used the IRRF7243 to make it possible to use my Logitech Harmony remote, same thing with this unit, poor IR range and choppy dimming. The stuff is expensive too. So I decided to build my own solution that replaced both these units.
I've used my own solution since June, and it has been working great! Response is faster, RF range is better, IR range is better and no more choppy dimming. Even the power line range is better, I think the power output of the XM10 is higher than that of the CM15PRO
Currently my solution supports all that the CM15PRO and IRRF7243 have to offer, with a few limitations.
- You have to write the scenarios in the Arduino Sketch. There is currently no API support, in my libraries, for uploading scenarios to the Arduino.
- The project I've published doesn't contain any real time clock implementation. I've written a library for the DS1307 chip and I'm considering adding it to the X10 project, but haven't had the time yet. If you want, send me a message and I'll e-mail you the DS1307 library.
- The .Net test app is not as user friendly and feature rich as the Marmitek Software. On the other hand I think my .Net library is a lot better than the Marmitek equivalent, and it's open source
- I've only tested the libraries with the European PLC adapter and RF remotes. But I'm pretty sure it will work with the American ones as well.
It would be really cool if you want to try this out! If you decide to, I'll try to help you out if you have any questions. And I'm curious about your iPhone app and your setup to make this work. I just ordered some Ethernet components, and I'm planning to have some sort of service interface running on the Arduino for iPhone/Android integration.
Your curtain opener/door lock is way cool, BTW.
-Thomas