Besides X10 and Zwave, the M1 also supports UPB, Insteon, Centralite (several flavors), RadioRA, and Vantage lighting systems. No "generic" ZigBee yet.
UPB requires a Powerline Interface Module (PIM) similar to X10. It intefaces to the M1 through an M1XSP module. 3 companies manufacture UPB...