It's been said before, but I guess it needs saying again:
Illinois - Chicago = Indiana
Remove Cook County and Bush beats Kerry 54-46. That's closer to Ohio than Indiana.
Illinois also has a handful of downstate areas such as the Quad Cities and the East St. Louis area that go Democratic in a 50-50 nationwide race. The only places in Indiana that are Democratic are the Indianapolis area (barely) and the Northwest part adjacent to Chicago. Also, the Chicago suburbs aren't as heavily Republican as their Indiana counterparts (look at how heavily Bush took the suburbs of Indianapolis). The three largest Chicago suburban counties - DuPage, Will, and Lake - went to Bush 54-45, 52-47, and 51-49 respectively. Should the trend continue, the Democratic Nominee in 2008 should have a decent chance at winning these suburban counties were (s)he to win the nationwide popular vote.