This entry on the Corner by Byron York helps to answer the question “Why Libby and not Armitage?” in the PlameGate thing:
Once the FBI started asking questions, in October 2003, Armitage told investigators he talked to Novak. Karl Rove told investigators he talked to Novak. The CIA's Bill Harlow told investigators he talked to Novak. Their stories, along with Novak's description of how he learned about Plame (Novak talked to investigators at the same time, describing the process, but not naming sources), all lined up pretty well.York dispels the notion there was some grand cabal at work with secret handshakes and blood oaths. In the end, it was all just CYA office politics with the State Department, the CIA, the White House and cast of dozens trying to remember (or mis-remember) their contacts with the media.
And then came Libby. Libby wasn't Novak's source. But in his interviews, Libby clearly raised eyebrows when he told investigators he learned about Plame from Tim Russert. According to the Libby indictment, Libby said that "Russert asked Libby if Libby was aware that Wilson's wife worked for the CIA." The story didn't fit with the others, and that would most likely make investigators suspect that somebody wasn't telling the truth. In this case, it probably appeared that person was Libby.