Recall these interesting historical facts in view of Libby's testimony to the the grand jury that Cheney told him that Bush had authorized the leaking of selective (and, by the way, misleading) information about the state of intelligence showing that Saddam Hussein intended to build a nuclear bomb. (What is misleading, of course, is the the National Intelligence Estimate that Bush apparently authorized the disclosure of said that while some intelligence indicated that the aluminum tubes might be used to enrich uranium, other intelligence (from the State Department and the Department of Energy) indicated the tubes were merely for launching conventional rockets--and Bush authorized only the release of the former information.)
Historical fact #1.
Fitzgerald wanted to interview both Bush and Cheney about the leaks of Valerie Plame's identity, but after much resistance (even though Bush had said he wanted to get to the bottom of the leaks) Bush allowed Fitzgerald a one-hour interview, not under oath, and only in the presence of Cheney.
Historical fact #2.
Bush consulted a criminal attorney, who attended the interview, and who insisted that the interview be limited to the leak of Plame's identity.
Historical fact #3.
Bush had personally seen a summary of the National Intelligence Estimate, which included the reservations about the intelligence concerning the aluminum tubes, and so the selective leaking was done by Bush knowing it didn't tell the whole picture about the tubes.
Historical fact #4.
When the fallacy of the Niger-uranium intelligence was coming to light because of Ambassador Wilson's revelations, that left only the aluminum tubes as evidence of Saddam's nuclear plans. Thus it became vital to reveal this information to the public in advance of the upcoming election, and to withhold the qualification of the intelligence--as part of the effort to shore up Bush's standing on the invasion of Iraq, along with the attack on Wilson's status.
So now the question becomes, did Fitzgerald, in his interview with Bush/Cheney ask any questions, and did the latter give any answers, that misled Fitzgerald in his investigation of the Plame leak? If he asked Bush/Cheney something like, "Was there any effort made by you to counter the adverse impression that Wilson was giving about Saddam's potential danger to the US"? and Bush/Cheney said "no," then that would be a lie, a lie that could have the effect of obstructing Fitzgerald's investigation into the source of the Plame leak. And that could be "obstruction of justice"--the same charge that brought Nixon down.