Yeah, the M1 has a special port onboard just for X10 PIMs but it's kinda irrelevant. You can just as easily add an XSP serial port to the M1 to tie in UPB/Z-Wave/Insteon just the same. Its true that the heart of the M1's lighting automation was built around X10 primarily but it's been adapted to work with the newer technologies. In fact, what you'll find is that the M1 has one set of lighting addresses it can handle - a total of 256 because that's how many total X10 device ID's you could have in a home. As they've adapted it they've created mappings of where ZWave or UPB devices fall into those same 250 based on their IDs. In fact, if you're running more than one technology, you have to take care to ensure there's no overlap in any device ID between them or you'll have multiple devices responding to the same command. I've very rarely seen this as an issue that comes up though - just because by the time someone adds a second protocol, they know their way around the system well enough to know the implications, and usually one technology is the majority of the devices, with the other being added to support specific things light door locks or thermostats keeping device numbers minimal.
I personally do still use a little X10 - but not the powerline, just the RF using a W800RF32 X10RF receiver to RS232 - I have that hooked into another automation controller translating commands into UPB so I can use some cheapo X10RF handheld remotes and motion sensors in a couple areas.
If you want to see more about the specifics of the lighting addresses, there's a lot of info in the XSP manual. You can google the PDF easily enough since you wouldn't have a login to the Elk site yet.
