The Congressional Research Service analysis was commissioned in response to the Wikileaks cable release. These highlights were published December 6, 2010 (and appear sans page numbers but sequential):

[W]e are aware of no case in which a publisher of information obtained through unauthorized disclosure by a government employee has been prosecuted for publishing it.

RE: disclosure by non-government employees:

The diplomatic cables obtained from State Department channels may also contain information relating to the national defense and thus be covered under the Espionage Act, but otherwise its disclosure by persons who are not government employees does not appear to be directly proscribed.

RE: treason

Because espionage is recognized as a form of treason, which generally applies only to persons who owe allegiance to the United States, it might be supposed that Congress did not regard it as a crime that could be committed by aliens with no connection to the United States.

RE: application to non-government officials

Application of the Espionage Act to persons who do not hold a position of trust with the government, outside of the classic espionage scenario (in which an agent of a foreign government delivers damaging information to such hostile government), has been controversial. The only known case of that type involved two pro-Israel lobbyists in Washington, Steven J. Rosen and Keith Weissman, associated with the American Israel Public Affairs Committee (AIPAC), who were indicted in 2005 for conspiracy to disclose national security secrets to unauthorized individuals, including Israeli officials, other AIPAC personnel, and a reporter for the Washington Post.

RE: extradition

Extradition is also generally limited to crimes identified in the relevant treaty... No U.S. extradition treaty currently in force lists espionage as an extraditable offense.

RE: Constitutional Issues

The publication of information pertaining to the national defense or foreign policy may serve the public interest by providing citizens with information necessary to shed light on the workings of government, but it seems widely accepted that the public release of at least some of such information poses a significant enough threat to the security of the nation that the public interest is better served by keeping it secret. The Constitution protects the public right to access government information and to express opinions regarding the functioning of the government, among other things, but it also charges the government with “providing for the common defense.” Policymakers are faced with the task of balancing these interests.

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... the Court has stated that “state action to punish the publication of truthful information seldom can satisfy constitutional standards...

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... if national security interests were not sufficient to outweigh the First Amendment principles implicated in the prior restraint of pure speech related to the public interest, as in the Pentagon Papers case,107 it is difficult to discern an obvious rationale for finding that punishing that same speech after it has already been disseminated nevertheless tilts the balance in favor of the government’s interest in protecting sensitive information.

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The Supreme Court has stated, however, that the question remains open whether the publication of unlawfully obtained information by the media can be punished consistent with the First Amendment. Thus, although unlawful acquisition of information might be subject to criminal prosecution with few First Amendment implications, the publication of that information remains protected. Whether the publication of national security information can be punished likely turns on the value of the information to the public weighed against the likelihood of identifiable harm to the national security, arguably a more difficult case for prosecutors to make.