> If you're running on a PC, there are hacks to 'borrow' the serial number of some ancient imac and have it work still (even ancient systems are still allowed to log into icloud, and those ancient systems didn't do any cryptography so all you need is a valid serial number that nobody else is using).
Hackintosh users have to do something similar to be able to sign into iCloud.
The serial number doesn't have to be valid (as in existing on a physical machine) to work, though. It just needs to look valid (be generated using the same methodology as real serials). In fact in order to prevent accidentally using a serial tied to a machine owned by somebody else, the recommended procedure is to generate a serial and check its AppleCare status to verify that it's not tied to a real machine, and if it is to regenerate and check until you find one that isn't.
Hackintosh users have to do something similar to be able to sign into iCloud.
The serial number doesn't have to be valid (as in existing on a physical machine) to work, though. It just needs to look valid (be generated using the same methodology as real serials). In fact in order to prevent accidentally using a serial tied to a machine owned by somebody else, the recommended procedure is to generate a serial and check its AppleCare status to verify that it's not tied to a real machine, and if it is to regenerate and check until you find one that isn't.