Apple began selling unlocked GSM iPhone 4 in the US, which arrive without a micro-SIM card so you can use them with any supported GSM carrier worldwide and without pledging to a lengthy service contract.
Per this support document highlighting iPhone carriers around the world, some or all carriers or authorized Apple distributors in each of the following 34 markets offer contract-free iPhones. However, the list lacks US since only Apple, and not carriers or local retailers, offers contract-free iPhone in its 200+ retail stores across the country.
The list includes: Belgium, Czech Republic, Finland, Germany, Greece, Italy, Luxembourg, Netherlands, Romania, Russia, Slovakia, Turkey, China, Hong Kong, India, Indonesia, South Korea, Macau, Malaysia, New Zealand, Singapore, Taiwan, Thailand, Vietnam, Brazil, Colombia, Ecuador, Mexico, Bahrain, Kuwait, Qatar, Saudi Arabia, United Arab Emirates and South Africa.
Neither carrier would loose subscribers to each other because their networks operate on incompatible radio technologies (GSM for AT&T, CDMA for Verizon). As a result, an officially unlocked AT&T iPhone cannot be taken to the Verizon network and vice versa.
AT&T has no immediate plans to offer authorized unlocking to existing iPhone 4 subscribers. As, the comapny said that "if you already own a locked iPhone 4, they are still not able to unlock it for you (at this time)."
Apple, Verizon Wireless did not respond.
Lacking carrier subsidies, prices are $649 for the 16GB model and a hundred more for its 32GB counterpart.