Apple now sells unlocked versions of all iPhone 4S models in their online store.
They start at $649 for the 16GB model and go up to $849 for the 64GB model. No CDMA roaming according to Apple, just GSM available on these phones. It just requires a microSIM to make it work from any GSM provider. If you want to get one without a contract see their website here: unlocked iPhone 4s.