The city had been required to drop its old logo, which used the Windy City skyline to represent a torch. Torches are apparently verboten for cities that aren't yet official Olympic host cities. (The location of the 2016 Games won't be picked until 2009).
Cities' Olympic logos can be really touchy subjects. Just ask London. That city's logo for the 2012 Games was roundly crticized when it was revealed earlier this year. (Personally, I actually really like it, although I do think this is pretty funny.)