These DMCA exemptions are always temporary and need to be renewed every three years. These exemptions are separate from those explicitly mentioned in the DMCA.
Thus, the 2010 exemptions -- which allowed unlocking -- are no longer valid. Notably, as well, rooting and jailbreaking of tablets is still) illegal.
Carriers can still offer unlock handsets themselves, whether post-purchase or by selling unlocked handsets (generally unsubsidized, as with the $299 / $349 Nexus 4 or the unlocked $649 16GB iPhone 5).
The decision was announced last October, with 90 days given for purchasers of new phones to unlock their devices.
To be clear, those who purchase used phones which were originally purchased prior to Jan. 26 can still unlock them. In addition, the change does not mean authorities will come after you for unlocking your phone. Instead, it means that consumers no longer have a "safe harbor" to use as a defense if they are sued for unlocking their handset.
Many groups, such as the Electronic Frontier Foundation (EFF) lobbied to keep the exemption in place, arguing that making unlocking illegal is anti-competitive and might produce more electronic waste since consumers might have to purchase a new handset to switch carriers (rather than, for example, unlocking their AT&T iPhone to move to T-Mobile).
The Library of Congress, however, decided that the DMCA exception is no longer needed because carrier rules regarding unlocking have been loosened, or so it believes. While that may be true, to an extent, it's also true that unlocking of cell phones negatively impacts big corporations such as AT&T.
Consumers have responded by starting a petition on the White House "We the People" website, asking for cell phone unlocking to be made legal. That petition has over 14,000 signatures as of late morning Saturday, Pacific time.
However, not only has unlocking been made more difficult, so has reaching the threshold necessary for an official White House response. Formerly 25,000, that number is now 100,000, meaning the petition has a long way to go before the 30-day deadline of Feb. 23.