I notice that most of the thread has gone the route of assuming GSM devices of some sort but if you want to do something that works for non-gsm too (e.g. N8x0 and N7x0) then I suspect a combination of the device serial number and product code with a 'salted' delimitation? e.g.

<product code><delimiter><serial number>

The delimiter is whatever you choose but I suspect the other two will be variable length. I'm sure other O/S will also have a means to determine these even if it's only the old *#06#