Never as architecturally distinguished as the former Sears Tower and the John Hancock Center, the Aon Center at least presented a vision of clean vertical lines, rising with aloof calm beside Grant Park.  But as sharp-eyed skyline watchers have noticed, a nasty little carbuncle has appeared on the roof of the 1,136-foot tower. About 85 feet tall, it resembles a cylinder stuck atop a flagpole. And people are wondering what it is...more