Following the news earlier that the iPhone 4S won't be available as an unlocked device until November, it's also worth noting that only customers on GSM carriers will be able to unlock the 4S.
Apple touted the iPhone 4S as a "world phone" capable of running on both GSM and CDMA networks, but as usual there seems to be more involved when it comes to the fine print. Unfortunately, an unlocked iPhone 4S will only work on GSM networks...