Up until now, Sprint's iPhone 4S was being sold to customers with the SIM card unlocked, meaning you could theoretically slide a non-Sprint SIM in there while overseas and avoid the carrier's standard roaming fees — but that's not happening anymore (at least, not without jumping through some customer service hoops). A new document obtained by SprintFeed suggests that starting today, the phones are being sold SIM-locked as you'd normally expect. Interestingly, customers who've already picked up and activated their 4S won't be impacted; in other words, their SIMs remain unlocked.
A little background is in order, though: Sprint had originally said that the iPhone 4S would start out SIM-unlocked, suggesting that an update would be pushed to lock those phones "shortly" after launch — so the fact that this document says existing iPhone customers won't be impacted is a notable change. If you need it unlocked, most major carriers have a policy in place that allows customers in good standing to go through a process and have that done, but if you buy a 4S from Sprint starting today, you'll need to call into customer service to kick off that process.