Assange would be prosecuted for committing the crime of possessing and releasing classified national security documents that could do this country harm. The First Amendment has no bearing whatsoever on whether Assange has committed this particular crime, so whether or not Assange is a "journalist" is irrelevant.
The problem here is that people get their information from the media, which is written by journalists, and journalists have spent the last half-century trying to persuade everyone that laws don't apply to them.
If a fully certified, bona fide, grade-A "journalist," rushing to get a story, swerves his car onto a sidewalk and mows down 20 pedestrians, he's committed a crime. It doesn't matter that he was engaged in the vital First Amendment-protected activity of news-gathering.
If Paul Krugman shoots his wife because she's talking too much when he's engaged in the First Amendment activity of finishing another silly column about the economy, he's committed a crime.
Journalists can't run red lights, they can't print Coca-Cola's secret formula, they can't torture sources for information, and -- as Gawker Media recently discovered when it published a story on the new iPhone before it was released -- journalists can't misappropriate lost property.
Fox News' Alan Colmes said he checked with Fox News legal analyst Andrew Napolitano, who told him there's no case against Assange because the government can't punish "the disseminator of information." They should have been on Gawker's legal team!
If Assange had unauthorized possession of any national defense document that he had reason to believe could be used to injure the United States, and he willfully communicated that to any person not entitled to receive it, Assange committed a felony, and it wouldn't matter if he were Lois Lane, my favorite reporter.
As I have noted previously, the only part of the criminal law that doesn't apply to reporters is the death penalty, at least since 2002, when the Supreme Court decided in Atkins v. Virginia that it's "cruel and unusual punishment" to execute the retarded.
Also, journalists can slander people at will. That ought to make them happy.