The Miami Herald explains that Deep Throat was probably a big lie:
The idea that Deep Throat is a fake -- or, at least, a composite constructed from several different sources -- is probably the single most widely held theory. Even Woodward's former literary agent, David Obst, has said the shadowy supersource was invented for the sake of showbiz: ''Without Deep Throat in All The President's Men, there's no book or movie,'' he wrote in his memoirs, adding that Deep Throat showed up in the manuscript only after the publisher rejected the first draft as too dull.
And if anyone was Deep Throat, it was likely to be Al Haig. Woodward was his buddy, and some of the leaks could have only come from Haig.