I'm repeating this post on this thread because I think it's an important point.

"As to whether his crime was so serious, Libby really had no way of knowing just what consequences he was setting in motion. Valerie Plame had been a covert CIA operative- (in fact, she was evidently brought home after Ames' perfidy became known for fear that she would be in danger); for all Libby/Cheney/Rove/Novak knew there were people down the line whose covers could have been blown or just possibly who could have lost their lives."