If you plan on traveling the world, you may be interested in picking up a Sprint or Verizon iPhone. Both carriers will unlock your phone if you need for traveling abroad.
This may be a major deciding factor for some as AT&T will not be doing this, meaning that world travelers will have to deal with expensive roaming rates.
According to MacWorld, this is because AT&T locks the micro-SIM so that the device can only be used on their network, while the others (Sprint and Verizon) use CDMA for service in the US and the micro-SIM slot is used for international service.
Here’s a bit of the report from MacWorld:
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.
Furthermore, they add that the Verizon iPhone will initially be locked, but they will unlock it for you. However, Sprint will sell it the iPhone 4S with the micro-SIM slot unlocked. Nice.