Apple officially laid the rumors to rest today when it began selling an unlocked version of its popular iPhone 4, enabling users to switch between networks at will.

The main advantages of an unlocked version of the iPhone are its lack of contract and its ability to switch carriers when traveling abroad. Rather than being tethered to a single network, international travelers can simply swap their SIM cards and avoid paying roaming fees.

Stateside, this feature may not be so advantageous, given that the only GSM carrier capable of supporting the iPhone 4 is AT&T. As of yet, an unlocked CDMA-supported iPhone 4, the model offered from Verizon, is still unavailable.

The unlocked iPhone 4 is available in 16-gigabyte and 32-gigabyte models from Apple's website as well as retail outlets. The smartphone starts at $649 – quite a bit higher than the handsets from AT&T and Verizon, though without the 2-year contract.

Additionally, the iPhone 4 comes with a number of mobile phone accessories, including earphones, a USB cable and USB power adapter.

With the success of the iPhone, Apple has significant strides in the smartphone market in recent years. A recent report by Nomura slated Apple to overtake Nokia in the market this quarter, moving into the No. 2 behind Samsung.