In 2010, the Librarian of Congress ruled that unlocking a phone to be able to move to another carrier was legal.   On 26th October 2012, the Librarian of Congress has changed his mind.  Unlocking phones purchased after January 2013 will be again illegal.

In the same ruling, the Librarian of Congress allowed the jailbreaking of iPhones for interoperability, but did forbid it for iPads!

Wireless telephone handsets – software interoperability
Computer programs that enable wireless telephone handsets to execute lawfully obtained  software applications, where circumvention is accomplished for the sole purpose of enabling interoperability of such applications with computer programs
on the telephone handset.
This exemption is a modification of the proponents’ proposal. It permits the circumvention of computer programs on mobile phones to enable interoperability of non-vendor-approved software applications (often referred to as “jailbreaking”),but does not apply to tablets – as had been requested by proponents – because the record did not support it.

Recently, the White House officially announced that it was

Time to Legalize Cell Phone Unlocking

How the White House will try to revert the Librarian ruling is unclear.

Once more, we see that interpretation of DMCA is complex and evolving with time.  Some decisions may even seem strange: authorizing mobile phone but not tablets (despite they use the same OS, and may act as phones), is difficult to understand for consumers.