US carriers selling the CDMA-enabled version of the iPhone 4S will unlock the SIM slot, so international roaming can be achieved by dropping a local SIM into the phone.
That's important as, despite Apple branding the 4S a "world phone", there aren't many roaming agreements between the CDMA and GSM networks that the 4S supports - so Sprint will just let you drop in any SIM you like, while Verizon told Macworld it would unlock the slot 90 days into a contract
CDMA networks are secured using an embedded element in the handset, as opposed to the removable SIM used by GSM networks, so a handset capable of moving between the two requires both elements.
4S Jesus mobes sold as GSM will never have their CDMA secure element activated, so will only work within GSM coverage (which is spreading, even in America), but customers who buy a 4S from Verizon or Sprint (US CDMA operators) will get a working secure element and a SIM, and the ability to replace that SIM with a local one when travelling.
That might provide an opportunity to make cheaper calls when travelling, at the expense of having to forward incoming calls to the local number. But, perhaps more importantly, it also reduces the imperative for Sprint and Verizon to run around the world signing roaming agreements with GSM operators with whom they've never spoken before.
Roaming agreements aren't just about technology, they're also about revenue cuts and carriage costs that are often just as complicated. Various forums exist for bulk deals, and both CDMA carriers will no doubt sign roaming deals with countries to which their customers often travel, but by leaving the slot unlocked it means the phone can use any GSM network in the world as long as the user has a pair of nail clippers handy (to cut a normal SIM down to an Apple-approved micro SIM).
What's less clear is if either Sprint of Verizon will permit customers to roam to one of America's GSM networks, such as AT&T. One might assume that it wouldn't be allowed, but given that the customer will be paying a subscription to the CDMA network anyway, and that the CDMA network will take precedence when it's available, then perhaps it's not as unlikely as it first appears.
Neither company is saying just yet, but with the handsets available imminently we'll soon find out. ®