One of the most interesting wrinkles in the iPhone 4S—which I review at length elsewhere—is its status as a “world phone.” Unlike previous generations, it supports both CDMA (Verizon, Sprint) and GSM (AT&T, the rest of the world) wireless technology right out of the box. But there are some interesting quirks to this product that may please savvy international travelers.

When Apple released the Verizon iPhone 4 in February, the iPhone 4's product line got a little more complicated. Because Verizon’s network in the U.S. uses a different wireless standard (CDMA) than AT&T does (GSM), Apple had to build different chipsets for each network, and thus, different phones. But the iPhone 4S changes all that: It can access either the CDMA or GSM network, depending on which carrier you sign up for.

For consumers on CDMA networks, this new phone design is a big deal when it comes to international travel. Users of iPhones locked to AT&T have always been able to travel internationally, as GSM is a cellular standard supported all over the world. It uses SIM cards (or, in the iPhone's case, micro-SIM cards) to talk to networks. When it came to the Verizon iPhone 4, however, users couldn’t travel internationally very reliably with their device, since it required a built-in device identification number on the CDMA network, which isn’t that widely used outside of North America and parts of Asia.

But the iPhone 4S supports both GSM and CDMA networks, and has a micro-SIM card slot no matter which carrier you subscribe to. So if you buy a phone from a CDMA carrier that doesn't use SIM cards, what’s in that slot? The answer is something that’s called a “roaming SIM.” If you're subscribed to a CDMA carrier in the states, when you travel internationally, your device can now switch to an international GSM network using a micro-SIM card. In other words, when outside the U.S., you can still get phone calls to your regular number and connect to the Internet, thanks to a micro-SIM that connects to Sprint and Verizon’s partner wireless networks in those countries—all at much higher rates than in the U.S., of course.

That’s essentially the experience AT&T iPhone customers have known all along. And because AT&T iPhones are locked to the micro-SIM card in the slot, when you travel overseas, you’re forced to pay AT&T’s roaming charges.

But there’s a new wrinkle that potentially makes the international-roaming experience better on Sprint and Verizon iPhones than it is on AT&T. Sprint plans to sell the iPhone 4S with its micro-SIM slot unlocked; Verizon’s will be initially locked, but if you’ve been a customer in good standing for 60 days, you can call Verizon and ask for an “international unlock.” (A Verizon spokesperson told me that this is Verizon’s standard policy for all world phones—it’s just the first time it’s manifested itself on an iPhone.)

So if you’re a Sprint or Verizon iPhone 4S customer traveling internationally, you can buy a pre-paid micro-SIM card with dramatically cheaper rates for data and voice calling, rather than pay for international roaming offered by U.S. carriers to their existing customers. (The only downside is that when you’re using some other carrier’s micro-SIM card, you’ll be using a local phone number rather than your phone’s U.S. phone number–so you won’t be able to receive calls unless you tell people to call the international phone number associated with your card, or use some third-party calling service like Google Voice.)

Advantage: Sprint and Verizon.