A report from yesterday claimed Sprint was going to sell the iPhone 4S with an unlocked SIM card, but a Sprint spokesperson has confirmed that report is false. Speaking to Ars Technica, the representative said Sprint will support international roaming, but only with a Sprint international rate plan. You can't pop in any micro-SIM from the least expensive carrier.
Verizon was also supposedly going to unlock the handset for customers after 60 days, but this report has not been corroborated. Verizon customers have the best shot at unlocking their iPhone 4S handsets as the carrier does unlock other global handsets like the Droid 2 Global. You have to be a customer in good standing with 60 days of service and you're allowed only one SIM unlock per line every 10 months. We've reached out to Verizon for an official statement and will update when we receive a response.
Update: Macworld received an official statement from Sprint that clarifies this situation. Their initial report was correct. Sprint will sell the iPhone 4S as an unlocked handset. Shortly after launch, an update will be pushed to the handset that will lock the phone. Customers who are in good standing and need international service can then request an unlock at a later date. The full statement can be read at Macworld.