So treason, like fascist or fire, is a word to be cried sparingly.
If Pvt. Manning did what he is alleged to have done, he will more likely be tried in a military court for violating at least two articles of the Uniform Code of Military Justice—Articles 92 ("failure to obey orders or regulations"), which can be punishable by court-martial, and 134, which carries a number of years in prison—as well as another Article 134 in the Manual for Courts-Martial, condemning personnel who "altered, concealed, removed, mutilated, obliterated, destroyed, or took ... a certain public record," including classified records. *
The UCMJ's Article 134 is a "general article" that covers a vast gamut of matters not specifically covered in other articles—"crimes and offenses not capital," among others. (Eugene Fidell, a lecturer at Yale Law School, says it's for this reason that 134 is known as "the fuck-you article.")
However, there are also criminal statutes in the civilian courts that could apply to Manning. There is, for instance, 18 USC 641, which outlaws the theft of public records. It carries a punishment of up to 10 years in prison—or, if the stolen documents are worth less than $1,000, up to one year. (How much are those WikiLeaks documents worth, and how would that be determined?)
More serious is 18 USC 793, aka the Espionage Act. It hasn't been invoked very often, but it has been trotted out on a few occasions in recent times.
In the mid-1980s, Samuel Loring Morison, a naval intelligence official, leaked secret satellite photos of Soviet shipbuilding facilities to Jane's Defence Weekly. (His motive was to show that the Soviets were building up their navy and thus move Congress to boost U.S. defense spending.) He was charged with espionage and theft of government property and spent two years in prison before President Bill Clinton pardoned him (at Sen. Daniel Patrick Moynihan's request), on the grounds that the law was sporadically enforced, that others had leaked similar material with no prosecution or even an official probe.
Just this year, Thomas Drake, a former senior executive with the National Security Agency, was prosecuted under Section 793(e), which prohibits "the willful retention of national defense information." (He leaked information, about what he considered to be a wasteful procurement contract, to a news reporter, but he wasn't charged with distributing the information, only with retaining it.) He was also charged with lying to an FBI agent and obstructing justice, but the impetus for the investigation was Section 793(e).
More telling still was the case of Lawrence Franklin, a Defense Department official arrested in 2005 for leaking classified information to two analysts with the American Israel Public Affairs Committee. He pleaded guilty and received a reduced sentence for agreeing to testify against the AIPAC analysts.
Those analysts, Steven J. Rosen and Keith Weissman, were charged, under the Espionage Act, with joining Franklin in a conspiracy (as the indictment put it)
to gather sensitive information, including classified information, relating to national defense, for subsequent unlawful communication, delivery and transmission to persons not entitled to receive it.