In a thoroughly useless corporate non-response to this question, on June 26, 2007, Apple added (and subsequently removed):
AT&T is the exclusive wireless carrier for iPhone in the United States. If you currently use another wireless carrier, you can choose to transfer your number when you activate your AT&T account.
In the original iPhone Q&A published on February 5, 2007, EveryiPhone.com noted:
It is a safe assumption that creative hackers will unlock the iPhone eventually regardless of Cingular's [now AT&T's] plans, but neither Apple nor Cingular have any plans to offer the iPhone unlocked at this time.
As expected, as spotted by Gizmodo, on July 18, 2007, hardworking hackers at the iPhone Dev Wiki (now offline) managed to partially unlock the iPhone and released a program called IASign for others to do so. Other hackers subsequently managed to completely unlock the iPhone using hardware and software methods on August 24, 2007, and on September 11, 2007, the original iPhone Dev Wiki team released a free software unlock with a GUI version a few days later.
Apple was not pleased by these unlocking methods, warned that future updates would disable the software on September 24, 2007, and sure enough, released a software update that disabled unlocked phones by the end of the week. The iPhone Dev Wiki team (now offline) found a way around this, Apple broke this method as well, and no doubt the cycle will continue indefinitely. On December 8, 2007, the iPhone Dev Wiki even open sourced the unlocking software so that the community could continue to upgrade and improve it.
As Apple was required to offer the original iPhone unlocked in France, EveryiPhone.com was hopeful that it would become easier and easier for those who don't mind either paying a premium or getting their hands a bit dirty to use the iPhone with a carrier or contract different from the one that Apple and the mobile service providers intend, and indeed, the original iPhone spread around the world and was used on any number of carriers.
For the iPhone 3G, AT&T became much more draconian and started requiring those who bought an iPhone in the US to activate it before leaving a physical Apple or AT&T store (online purchase was no longer offered -- except for business users).
Some time "in the future", AT&T reportedly would allow one to purchase an iPhone 3G without a two-year contract for US$599 and US$699 for the 8 GB and 16 GB configurations, respectively, but these "probably still would be locked to the carrier". Sure enough, starting March 26, 2009 -- in an effort to reduce inventory in preparation for future models -- the company did just that and the phones did indeed remain locked to the carrier.
Nevertheless, the version 2.0 software pre-installed on the iPhone 3G was unlocked and jailbroken within hours of its release, so other GSM-supporting carriers (T-Mobile) were available for those in the US willing to break a contract and pay an early termination fee or buy a "never locked" iPhone 3G that had been re-imported from a market that allows the sale of such iPhones.
For the iPhone 3GS and iPhone OS 3.0, hardworking hackers went to work and quickly released tools called UltraSn0w and Purplesn0w that make it simple to unlock locked iPhone models running the latest version of the operating system. This doesn't eliminate contractual obligations, of course, but it means that unofficially unlocked iPhones will continue to be available through gray market channels.
For the iPhone 4 (GSM) and iPhone 4 (CDMA) models, hardworking hackers also have released an assortment of jailbreaking tools and the cat-and-mouse game between Apple and unauthorized unlockers continues.
For those interested in buying an unlocked iPhone, it is strongly recommended that one buy from an authorized reseller outside of the US, preferably in person, to avoid potential issues in the future related to unofficial unlocking methods. Theoretically, unofficial "hack" methods could compromise data security and Apple or a carrier could remotely disable the device entirely as well.
Americans should note that the world is full of beautiful countries that do not lock the iPhone to a carrier from Belgium to the Czech Republic, Hong Kong to New Zealand, along with many others, so consider picking up an iPhone while on vacation. Naturally, be sure to verify that the iPhone 4 is for sale in the country of interest and has officially unlocked models in stock prior to departure. Bon voyage!
