Carol Leonnig and Jim VandeHei state the obvious: if Libby is not insane, his perjury is intended to protect Cheney in some way:

Libby May Have Tried to Mask Cheney's Role: In the opening days of the CIA leak investigation in early October 2003, FBI agents... had something that law enforcement officials would later describe as their "guidebook"... the daily, diary-like notes compiled by I. Lewis "Scooter" Libby....

The investigators had much of this information before they sat down with Libby on Oct. 14, 2003, and first heard from him what prosecutors now allege was a demonstrably false version.... Libby said that, when he told other reporters about the CIA operative and her marriage to Iraq war critic Joseph C. Wilson IV, he believed he had first learned the information from Tim Russert of NBC News and was merely passing along journalistic hearsay.... In the aftermath of Libby's recent five-count indictment, this curious sequence raises a question of motives that hangs over the investigation: Why would an experienced lawyer and government official such as Libby leave himself so exposed to prosecutor Patrick J. Fitzgerald? Libby... gave a false story... even though he knew investigators had his notes, and presumably knew that several of his White House colleagues had already provided testimony and documentary evidence that would undercut his own story....

The vice president is shown by the indictment to be aware of and interested in Plame and her CIA status long before her cover was blown.... White House aides privately wonder whether Libby was seeking to protect Cheney from political embarrassment. One of them noted with resignation, "Obviously, the indictment speaks for itself."...

[T]o Libby's defenders, the timing of Libby's alleged lies supports his claims of innocence. They say it would be supremely illogical for an intelligent and highly experienced lawyer to mislead the FBI or grand jury if he knew the jurors had evidence that would expose his falsehoods. Libby, they say, is guilty of nothing more than a foggy memory and recollections that differ, however dramatically, from those of several witnesses in the nearly two-year-old investigation....

Libby... during his first FBI interview... said he believed that he first learned about Plame on July 10 or July 11, 2003, in a conversation with Russert. ... Libby did not seek to deny that he had learned about the Plame link from Cheney -- as revealed by Libby's own notes -- but simply said it had slipped his mind that the vice president was an earlier source of the information than Russert, lawyers familiar with the case said.... Fleischer reportedly told investigators that, at a lunch on Monday, July 7, Libby told him that Wilson's wife worked at the CIA and confided that the information was not widely known. Fitzgerald, in announcing the indictment two weeks ago, called attention to this conversation with Fleischer to show how improbable he regarded Libby's account: "What's important about that is that Mr. Libby . . . was telling Mr. Fleischer something on Monday that he claims to have learned on Thursday."... Fitzgerald got all the reporters' testimony that he had sought. Russert, Time magazine reporter Matthew Cooper and Judith Miller of the New York Times all testified about their conversations with Libby. All contradicted Libby.