According to a leaked internal Sprint memo, all Sprint iPhone 4S handsets sold starting today will have their SIM card slots locked to Sprint. It has never been fully clear how Sprint was shipping its iPhone 4S models but it now looks like they were originally shipping them with unlocked SIMS.
According to the memo, the SIM locking will be done during the iPhone’s activation process and it will be done invisibly. The customer will not see any notification of it happening.