Here's the Vanity Fair article unmasking Deep Throat, the secret source for Bob Woodward and Carl Bernstein, as W. Mark Felt, the #2 man at the FBI during the early 1970s. The Washington Post has confirmed the accuracy of the story.
I remember reading this book in the ealry 1990s while I was in journalism school and being persuaded that Alexander Haig was probably Deep Throat, but so much for that theory. It might be interesting to go back and re-read All the President's Men to see how likely it is that Felt would have known the information that Woodward and Bernstein attribute to him.
UPDATE (6/1): Speaking of General "I am in charge" Haig, he says that he isn't surprised that Felts was Deep Throat: " Former Secretary of State Alexander Haig told WPBF News 25 that he suspected W. Mark Felt all along."
Hmm, sounds a little like revisionist history to me. Considering that Haig was frequently thought to have been Deep Throat, if he really had strong suspicions that Felt was the guy, doesn't it seem like he would have pointed the press in Felt's direction? By no means was I a Watergate-expert, but I did spend some time reading about it when I was in J-school, and I read stories listing the possible Deep Throats, and I don't remember Felts' name.