I've had long discussions with Crestron and Control4 and AMX professionals, and have come to the conclusion that the various members of the Zigbee consortium don't WANT a computer that's not Crestron, AMX, Control4, etc, to be able to control ZigBee devices. The whole standard has been a shoddy slip-shot from the beginning, only recently gaining the equivalent of standardized command classes....but the simple truth is, that ZigBee computer interfaces for off the shelf computers do not exist.
-Thom