http://www.msnbc.msn.com/id/38413597/ns/technology_and_science-wireless/ From NPR
The decision to allow the practice commonly known as "jailbreaking" is one of a handful of new exemptions from a federal law that prohibits the circumvention of technical measures that control access to copyrighted works. Every three years, the Library of Congress authorizes such exemptions to ensure that existing law does not prevent non-infringing use of copyrighted material.
Both previous and current such rulings can be found here. U.S. Copyright Office
With the following being the exemption noted on MSNBC and NPR.
Computer programs that enable wireless telephone handsets to execute software applications, where circumvention is accomplished for the sole purpose of enabling interoperability of such applications, when they have been lawfully obtained, with computer programs on the telephone handset.