When rocker John Mellencamp performed for the recovering soldiers at Walter Reed Army Medical Center on Friday night, a couple of things were missing. He squelched his typically blistering rhetoric against the war in Iraq. Also MIA, as it turned out, was folkie and antiwar activist Joan Baez, who says she was disinvited from the event by Army officials. In a letter that appears today in The Washington Post, Baez says Mellencamp had wanted her to perform with him and that she had accepted his invitation. "I have always been an advocate for nonviolence," she writes, "and I have stood...