Some will bring up the cases where no crime was committed but there are other reasons to hide the truth from a prosecutor. The most famous example is probably Bill Clinton, who clearly did not commit a crime with regards to Monica Lewinsky, but lied about his involvement with her to avoid scandal (worked out great, didn't it?) Many, including myself, believe that the prosecutor asked questions designed to embarrass Mr. Clinton rather than get at any true criminal activity. In this sort of situation, then, is it acceptable to lie? Obviously not. If claiming prosecutorial bias were an adequate excuse for giving false information then everyone would make said claim (and many do.) You can't trust them when they say they did nothing illegal because if they had they would have covered it up. If you can't get someone for their crimes then it makes perfect sense to nail them for the cover up. That's WHY you can't get them for the crimes. The two things are not unconnected.
Did Scooter Libby commit a crime? I don't know. Did anyone else in the White House? I don't know. Do I trust them when they say they haven't? No. Is it possible that Plame's name was revealed by someone who didn't know she was undercover? Yes. Was it still an incredibly sleazy thing to do? Undoubtably.
Should someone go to jail for these actions? That will probably come out during the trial. Should Scooter Libby do time if it's found he obstructed justice?
Why the hell not?