From 26 of January 2013 (which happens to be today) unlocking an iPhone will require carrier permission, as it is going to be “illegal” to unlock without that in the US from now on. The practice of using device with another network is not easy and cheap anymore and may become impossible as well if you decide to stick on the legal route.
The law to make the process of unlocking has been in consideration since October when Library of Congress’ Register of Copyrights suggested that the process of unlocking smartphones including the iPhone without carrier permission should be stated illegal. The law is applied to the iPhones bought after today, that is 26 January 2013.
Generally, to unlock an iPhone, you need to have a code or company services that can help you to remove the limitations on your device that attempt to forbid you to open some particular carrier.
An unlocked iPhone can be used with any compatible network. Shifting to other networks had been a useful practice for the people traveling overseas as they could simply change the SIM card and enjoy local charges and for those who wanted to change carrier along with fulfilling the contract with their previous carrier.
Also major carriers for example AT&T will unlock your device as the contract gets over. For iPhone 5, Verizon offers that it will unlock the device out of box straight away. So it isn’t a huge deal for many users.
An interesting fact is, even White House has submitted a petition to Librarian of Congress to repeal the decision.
It is still not clear that how will this law be implemented. However it is pretty clear that both offline and online third party unlocking services need to shut down or a legal action will be taken against them if they still continue.