The lie has to be material to the investigation.
If they asked him what he had for lunch, and he said "a hot dog," when in fact he'd had a chicken sandwich (and knew he was lying!), there is no crime.
This investigation was centered on who learned what about Plame, from whom, and when. The indictment charges that Libby lied about how and when HE learned about Plame.
Okay, I'll defer to you. I am manifestly unfamiliar with grand jury proceedings and such. My larger point is that we need to be careful not to advance a double standard just because this is some Republican. Sure, the whole investigation is stupid, but lying under oath is lying under oath. But if you say there's a legal difference, okay.