UPDATE: A Sprint representative tells Ars Technica that it will not offer the iPhone 4S with an unlocked micro-SIM card slot, despite earlier reports that it would. Instead, customers must use Sprint’s international GSM roaming service. The Sprint spokesperson also said that Verizon’s device likely works the same way. There is no evidence that AT&T will offer its version unlocked either. In other words, if you buy the iPhone 4S from any US carrier, the device’s true ‘world phone’ capabilities are rendered nearly useless. Cue customer outrage in 3…2…1…

Original report:

In a first for US wireless carriers, Sprint and Verizon Wireless will allow iPhone 4S customers to unlock their mirco-SIM card slots, which enables use on wireless networks around the world, reports Jason Snell at MacWorld. Sprint will simply sell the iPhone 4S with its micro-SIM card slot unlocked. Verizon, on the other hand, will remove its carrier lock if the customer has been with them for 60 days in good standing, and explicitly requests an “international unlock.” It is not yet clear whether AT&T will allow international roaming, something they have traditionally forbid.

Unlike past versions of the iPhone, Apple’s iPhone 4S, which hits stores on Friday, is a true “world phone,” which means it can technically connect to both CDMA (Sprint and Verizon) and GSM (AT&T and most International carriers) wireless radio signals. Since all iPhone 4S units have micro-SIM cards, those purchased for use on Sprint or Verizon come with a “roaming SIM,” which allows users to connect to Sprint and Verizon’s partner networks overseas, and send and receive calls from their US number.

Using the phone this way comes at an exorbitant cost – something AT&T customers who’ve tried to use past iPhone models overseas know all too well. Because AT&T uses a GSM signal, iPhone users have long been technically able to use their devices overseas. But this also required using international roaming, and could regularly cost users thousands of dollars, especially if they tried to use the Internet or email.

With a carrier unlocked SIM, iPhone 4S users will be able to swap in a pre-paid SIM card – a common practice for international travelers who want to use their handset overseas – which will give them a different, local phone number, and the ability to connect to local networks at regular rates, as opposed to the jacked-up roaming rates that come with using the “roaming SIM.”

By removing the carrier locks for their iPhone 4S customers, Sprint and Verizon have put the pressure on AT&T to follow suit.

Customers can purchase an unlocked iPhone 4, or an iPhone 4S, from Apple – but they’ll have to pay full price for the device. Apple can also, in certain instances, override the carrier lock to enable the world phone functionality.