It seems Col. Boylan, director of the military's Combined Press Information Center in Baghdad, sends an unsolicted email from Iraq to Greenwald and then denies ever sending it. When Greenwald emails Col. Boylan back in an attempt to clarify whether the military's email has been hacked....Boylan basically says don't worry about it.
You need to read the whole thing to appreciate the weirdness of the affair. An overwhelming amount of evidence (in my opinion) points to Col. Boylan outright lying about sending the email.
UPDATE: Another nice summary and commentary on the Greenwald/Boylan mystery email here at Salon.
What's most bizarre about this story is the military's determined inattention to it -- a man whose job is to be the mouthpiece for the war's top general is claiming that his words are not his own, and authorities are nonchalant. On Tuesday morning I called the Multi-National Force in Iraq, and I got through to Cmdr. Scott Rye of the Navy, the day chief of media operations. Rye had not heard of the controversy. He said that he takes Boylan "at his word" that he was not the author of the message, but added that he's interested in looking into whether someone has been impersonating the colonel. I sent Rye the disputed e-mail. He responded: "I'll share with the IT guys and see what, if anything, they can determine." I have not heard back.