Thanks for sharing!
I recall reading that someone used one of the smoke detectors that has a safety lamp which activates when in alarm state. They connected the coil of a reed relay in parallel with the leads to the lamp so that the relay's contacts would close when the alarm (and thus the lamp) activates. The relay's contacts were then wired to the PowerFlash module and it was set to input type "B". It is nice to know that there is an even simpler method that is tested and works when one has the type of smoke detector with the signal wire that links multiple detectors together. (And I didn't realize that the signal was just a 9V DC pulse.) However, the relay method is a little safer if you can mount the relay in the detector because then you will only need a dry contact interface which doesn't carry line current to the PowerFlash module. As Brian hinted, the 9V DC signal is referenced to AC neutral and is a shock hazard if any connectors are exposed.
Do you know whether the 9V signal is present on the red wire when the smoke detector is in alarm state but there is no AC power supply (power is out) and it is running on the backup battery? It won't help you if the PowerFlash module and whatever X10 receiver you are using also don't have power, but I am planning to put my automation PC on UPS power and it would be great to still get a signal from smoke detectors this way (via optoisolated input, not X10) shortly after a power outage.