Jim VandeHei and Walter Pincus report that lawyers close to the Valerie Plame investigation (identified as lawyers for witnesses, but presumably those would be potential defendants) say that Patrick Fitzgerald is pointing toward conspiracy charges.

"Many lawyers in the case have been skeptical that Fitzgerald has the evidence to prove a violation of the Intelligence Identities Protection Act, which is the complicated crime he first set out to investigate, and which requires showing that government officials knew an operative had covert status and intentionally leaked the operative's identity.
But a new theory about Fitzgerald's aim has emerged in recent weeks from two lawyers who have had extensive conversations with the prosecutor while representing witnesses in the case. They surmise that Fitzgerald is considering whether he can bring charges of a criminal conspiracy perpetrated by a group of senior Bush administration officials. Under this legal tactic, Fitzgerald would attempt to establish that at least two or more officials agreed to take affirmative steps to discredit and retaliate against Wilson and leak sensitive government information about his wife. To prove a criminal conspiracy, the actions need not have been criminal, but conspirators must have had a criminal purpose."

Note first that everything VandeHei and Pincus report about conspiracy is from defense lawyers. They may be spinning, and Fitzgerald may not be showing all his cards. I'm not sure where VandeHei and Pincus get the idea that the Intelligence Identities Protection Act is the crime Fitzgerald "set out to investigate." He set out, as I recall, to investigate what crimes might have been committed in the course of unmasking Valerie Plame Wilson's identity as a CIA operative who once worked under Non-Official Cover. The potential defendants and their friends in the White House and the conservative media have been working hard to keep everyone focused on the hard-to-violate IIPA rather than the easier-to-violate Espionage Act.
The second paragraph quoted above seems to me somewhat confusing about the meaning of "conspiracy" as a criminal charge. A "criminal purpose" means either the purpose to do something forbidden by some substantive criminal law or an "attempt to defraud the United States."

Here's the full text of the relevant statute, 18 U.S.C. 371
Section 371. Conspiracy to commit offense or to defraud United States
"If two or more persons conspire either to commit any offense against the United States, or to defraud the United States, or any agency thereof in any manner or for any purpose, and one or more of such persons do any act to effect the object of the conspiracy, each shall be fined under this title or imprisoned not more than five years, or both.