Who said anything about cloning phones? I said GSM, you put your SIM into any phone you want and it works. The carrier doesn't care.
In my case, I have a GAIT multi-mode AMPS/TDMA/GSM phone, so Cingular knows what my ESN is, but if I stick my SIM into another GSM phone, it works fine. Indeed, they tell me to do that if I want to roam in a900/1800 country.