Yes, it is. I heard some talking heads on one of the news channels asking why Libby would lie to the FBI and Grand Jury when it was clear that there was documentation otherwise. I guess he could argue on exactly what "knew she worked at the CIA" means, ie. he never saw her employee badge, but that's Clintonesque.
I guess he thought that he could deflect the FBI investigation and that it would blow over. Once he tried to do that, he couldn't contradict his FBI statements in the later Grand Jury. Still, the guy's no dummy, he had to know that all the details would be found out.
Well, no it isn't. There are people who work for the CIA openly; there are people that work for the CIA quietly; and there are actual deep cover agents. It is not even remotely illegal to tell people about the first; only in extreme circumstances illegal for the second; and only sometimes illegal to tell people about the third. ...And knowingly telling that they are part of the third group is only illegal under certain circumstances.
Regardless, the law that protects the identity of a deep cover agent is NOT intended to act as a shield against their participation in a lie such as Wilson made in misattributing why he went to Niger. Her name was a direct answer to a point of the debate, and she was not in a physical location where she was under the unprotected control of a foreign country - which is what the law was about.