Update: A Sprint spokesperson has said that our source article was incorrect, telling us that its SIM will not be unlocked and adding that the card will not even be removable:
Update 2: We heard from Sprint a second time, which redacted part of its previous statement. The carrier further explained "the SIM is removable and is not affixed to the device," which as you may notice directly contradicts the statement made earlier today. We've reached out to Apple twice and are awaiting further clarification.
"Our SIM does not come out of the device - I believe the same is true of Verizon's iPhone but you would need to confirm that with them. Customers can sign up for one of our international rate plans and use this phone all over the world. When traveling internationally, there is a setting that must be turned on within the device to connect to GSM. The phone will work with a SIM that is provided within the device out of the box. International voice and data charges are on a pay-as-you-go basis and vary based on the country where the customer is using their phone; a list of rates is available at www.sprint.com/international."
Update 3: A Verizon spokesperson has confirmed that the original version of the article is in fact correct:
"The iPhone 4S works like all of our global phones. The phone comes in box with a SIM locked to our network and the network of our roaming partners including Vodafone, one of our parent companies. Customers sign up for a Verizon Wireless voice plan (beginning at $39.99 for 450 min.) and a data plan beginning at 2GB for $30. When they choose to go out of the country, they should call Verizon Wireless and sign up for a global plan. The customer can also request to have the SIM unlocked if they want to use a local service while out of the country. The request needs to be made prior to leaving the country, their bill must be current and the phone must be active on the account for 60 days before we will unlock the SIM."