It isn't really an easter-egg, it is more of a hack to get them to work. The CDMA part doesn't rely on a SIM at all, however the GSM part does. In the US, the GSM bands are locked out for other US carriers, so the Chinese ROM is used to unlock those bands. Outside of the US, this hack should not be necessary assuming that you got the phone SIM unlocked. As far as other phones, I know that the Droid 2 Global also features both GSM and CDMA circuitry, though I don't know if there's a hack for that one with a Chinese ROM. There likely are other models out there that are capable of both CDMA and GSM, but I don't know any off hand.