Duval County (Jacksonville) is a place of some strange happenings -- it has the only Northern-flowing major river in America for instance -- but check out its bizarro voting shifts. In 2004, Bush won 58-42% there; he won 220K votes to John Kerry's 158K. But in 2008, the county split about 50-50%. McCain won 188K votes and Obama took 186K -- that's a 32,000 vote shift.
That could be two things at work: Either A) energized African-Americans, or B) a depressed Republican base. But in my experience with Jacksonville, there aren't a lot of swing voters there. It's not very suburban, where I think most swing voters live, and it's very Southern (read: racially polarized on voting preferences).