Kevin Drum puts it best:
For now, here's the bottom line: Fitzgerald didn't charge Scooter Libby with mistakenly making a few unimportant false statements to the grand jury. He charged him with deliberately constructing a false story about how he learned about Valerie Plame, and then repeatedly telling this story to both FBI agents and the grand jury. That story was a lie, and it was a premeditated lie designed to cover up the fact that he had engaged in a long and persistent effort to uncover information about Joe Wilson's wife and disseminate it to reporters.
Libby could have told the truth, but then he would have had to admit his role in outing a CIA agent in order to score political points against a critic of the administration. He didn't want that campaign to become public, so he invented a cover story, repeated it under oath, and stuck to it on multiple occasions.
It's serious stuff.