W. Mark Felt is Deep Throat [Updated]
Or so he and his family say. He was the #2 FBI man. He's also 91 years old and may have just been asked if he was the anonymous source for so long that now he believes it. Woodward and Bernstein are keeping their mouths shut. The Vanity Fair piece breaking it is a pdf file.

[Update: Josh Marshall notes that the Washington Post has confirmed the story with Woodward, Bernstein and Bradley.]