Now the Machine ID in my plugins is based on the serial number of the first hard disc in the computer, the brand string of the main CPU, the serial number of a Mac or it's model name. Those 4 values typically only change if you replace the harddisc or get a new computer. All the functions we use are exposed in the SystemInformationMBS module, so check them. You can build your own ID yourself if you like.
We do not use the MAC Address for the ID as it changes if you switch between wireless network and wired network. Also not counted in is the physical memory size as people like to upgrade their memory and the ID should not break too often.
More information: http://www.monkeybreadsoftware.net/pluginpart-systeminformation.shtml