There has been an exception made to the DMCA that now allows users to unlock their mobile devices. From EFF:

San Francisco - The Electronic Frontier Foundation (EFF) won three critical exemptions to the Digital Millennium Copyright Act (DMCA) anticircumvention provisions today, carving out new legal protections for consumers who modify their cell phones and artists who remix videos — people who, until now, could have been sued for their non-infringing or fair use activities.

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In its reasoning in favor of EFF's jailbreaking exemption, the Copyright Office rejected Apple's claim that copyright law prevents people from installing unapproved programs on iPhones: "When one jailbreaks a smartphone in order to make the operating system on that phone interoperable with an independently created application that has not been approved by the maker of the smartphone or the maker of its operating system, the modifications that are made purely for the purpose of such interoperability are fair uses."
http://www.eff.org/press/archives/2010/07/26