The word is that Deep Throat (whoever he is) is gravely ill. John Dean suggests that we will know very soon (and apparently it is not Mr. Dean).
One suggestion I haven't seen elsewhere is E Howard Hunt. Mr. Hunt has been in poor health for some time now. According to an October 2004 Slate interview, Howard Hunt is suffering from lymphoma and atherosclerosis and is wheelchair-bound.
And no, this has nothing at all to do with Mr Hunt being one of my Dead Pool picks.
For the record, it should be noted that the Wikipedia entry on Deep Throat states that Dean's claim has been denied by Woodward and the current Post editor.