Knowing, then knowing and then doing something powerfully useful with it is your next challenge Mr. Olbermann

Knows! But also knows that if he tells what he knows, his credibility will be questioned, he will be character assassinated like Spitzer and Blagojevich (BTW nobody character assassinates like the American media. It's blood sport.) and he will have to live the rest of his life as an anonymous has-been.
Maybe it's better for someone in his position to chip away judiciously and systematically at the false reality and therefore keep his job. Let's face it, if he's gone there aren't too many who are willing to go as far as Keith goes.
It's a fine balancing act Keith has to maintain and he is brilliant enough to pull it off. He was the most consistent and loudest critic of the Bush administration and probably opened the eyes of many Americans who were otherwise subject to the typical pro Bush propaganda in the rest of the media.
Hopefully a time will come when it won't be considered heresy to suggest we haven't been told the whole truth about 9/11. Then maybe Keith and others like him will feel safe to push the issue.