Under 17 USC Section 1201(a)(1), the Librarian of Congress is required every three years to determine whether there are any classes of works that are exempted from the prohibition against circumvention of technology that effectively controls access to a copyrighted work. This provision is part of the Digital Millenium Copyright Act ("DMCA").
On July 26, 2010 the Librarian of Congress announced that among the six classes of exempted works, one includes:
"Computer programs, in the form of firmware or software, that enable used wireless telephone handsets to connect to a wireless telecommunications network, when circumvention is initiated by the owner of the copy of the computer program solely in order to connect to a wireless telecommunications network and access to the network is authorized by the operator of the network."
This exemption is important and may significantly effect wireless telephones such as iPhone, which attempt to restrict users' iPhone or iPad access exclusively to the Apple App Store.
This is a link to the US Copyright Office Announcement.