Not so surprising for a newspaper to do so, but it is pretty surprising to see this newspaper do so. The Tribune has never before endorsed the Democratic candidate for President. In fact, its history as a staunchly conservative newspaper makes this endorsement even stronger.
The endorsement itself is a litany of how McCain has failed to capture the hearts of anyone but the most obtuse of Republicans, from his shifting policy on taxes to his selfish choice for Vice President. It is also a first-hand view of the experience and abilities of Obama from the paper and staff that have watched his political career from the beginning.
Whatever your leanings, it's a cogent argument and a heavily-weighed endorsement; read it here.