Since its release, Sprint has skirted around any discussion regarding the locked/unlocked status of the iPhone 4S SIM slot. Apparently the Sprint iPhone 4S SIM slots have been unlocked, but according to SprintFeed they'll be locked starting today.
The Sprint memo says that the SIM lock occurs during activation that this policy won't affect iPhone 4Ses that have already been activated (for now, at least). Those with their SIM slots locked will have to call Sprint's customer relations to have it unlocked if need be. [SprintFeed via MacRumors]