Originally Posted by glennj
An iPhone that runs on Verizon's network, instead of ATT's, does not have a GSM radio at all and cannot accept a SIM card of any type. It cannot ever be used on anything but CDMA, so no - you cannot do anything to a Verizon version iPhone to make it work on ATT or T-Mobile or any other GSM network.
Unlike the Verizon iPhone 4, the Verizon iPhone 4s does
have a gsm radio and micro sim card slot, and can be used internationally - like any Verizon world phone, it has a Vodafone sim built-in and can be unlocked by a customer with 60 days of service. However, in this case that doesn't matter much, because I believe it is still locked in the US and cannot be activated on a US carrier, prepaid or postpaid.
However, if it was bought off-contract, at full price, the sim should be unlockable.