According to a report Sprint is planning to lock their Apple iPhone 4S starting today (11/11/11). The process will happen during activation and is invisible to both the customer and the seller.All devices purchased from Sprint to date were sold unlocked, so customers who purchased the iPhone 4S before today will not be affected. Those who purchased the device earlier would have the freedom to use their micro-SIM slot freely when roaming internationally.
source - SprintFeed