Apple will offer the iPhone 4S in an unlocked flavor starting from November in the United States. This is nothing new for most other countries who can already pre-order an unlocked iPhone 4S today.
The unlocked iPhone works only on supported GSM networks, such as AT&T in the U.S. When you travel internationally, you can also use a micro-SIM card from a local GSM carrier. The unlocked iPhone 4 will not work with CDMA carriers such as Verizon Wireless or Sprint. If you don’t want a multiyear service contract or if you prefer to use a local carrier when traveling abroad, the unlocked iPhone is the best choice. It arrives without a micro-SIM card, so you’ll need an active micro-SIM card from any supported GSM carrier worldwide. To start using it, simply insert the micro-SIM card into the slot on your iPhone and turn it on by pressing and holding the On/Off button for a few seconds. Then follow the onscreen instructions to set up your iPhone.
The benefit of buying an unlocked iPhone is mostly to those that travel. The ability to buy a local SIM card and avoid the heavy roaming charges is a huge benefit to some. The downside is the price of the handset. Apple will be selling them for $649 for the 16GB, $749 for the 32GB and finally a whopping $849 for the 64GB. It is GSM only and should work even with T-Mobile but only with Edge; don't expect any 3G data!