Following on from a ruling by the Librarian of Congress three months ago, today represents the day after which it becomes illegal to unlock a cellphone that is still contracted to a carrier in the USA. In the context of purchasing an iPhone from a US carrier like AT&T, this means that after today, if users should purchase an iPhone locked to the carrier, it is now no longer allowed to unlock that device from the network using third party methods.
It is worth noting however, that there are several exceptions to this rule. The carrier Verizon, for instance, supplies iPhones that are already unlocked and it is, of course, possible to purchase a factory unlocked iPhone straight from Apple. In addition, unlocks can, and will, still be carried out once a cellphone goes off contract and the consumers is free from the minimum period. Furthermore, the ruling does not apply to iPhones and other devices that were purchased prior to today. Also, the ruling only applies to devices in the United States.
The timing of the implementation of this ruling is particularly significant as Apple is expected to release iOS 6.1 in the coming weeks, which will likely be followed days after by an iOS 6.1 untethered jailbreak, giving users the opportunity to unlock a device like an iPhone 5.
The process of unlocking refers to the process of lifting the restrictions placed on a cellular device that only allow it to work with one specific network. This is separate from the process of jailbreaking to which the ruling does not apply.