Apple now starts selling unlocked iPhone4. Before that iPhone was only available with contract to wireless carriers like AT&T and Verizon. According to Apple unlocked iPhone4 has all the features like FaceTime video calling, Retina display, HD video recording, and more which Apple provide with wireless carriers.
The unlocked iPhone 4 requires an active micro-SIM card that you get from a supported GSM wireless carrier. Simply insert the micro-SIM card into the slot on your iPhone, connect to your PC using the USB cable, and launch iTunes on your computer to complete the setup.
Currently 16GB and 32GB unlocked models available in $649 and $749 in both white or black color. If you are not from US and still want to buy unlocked iPhone4 than you can buy using a package forwarding service.