The Librarian of Congress, decided that unlocking cellphones would no longer be allowed and will be deemed illegal (October 2012). However there was a 90 day window during which people could buy a phone and unlock it. That window is now closed (officially on Jan 26).
Though unlocking is illegal, people can still jailbreak (for installing new software that is not possible without jailbreaking). Unlocking a phone helps in freeing up the restrictions that keep the device from working on more than one carrier’s network. Frequent international travelers will always prefer to have an unlocked phone. Now people will not have the freedom of being able to switch carriers.
Though there are excepts like Verizon’s iPhone 5 which comes out of the box already unlocked, and AT&T will unlock a phone once it is out of contract. As far the rest of the users are concerned, they now need to pay the full price of the mobile to unlock or talk to their service providers.
P.S: There is a petition filed to the US President against this law.