There's nothing bold about doing something when it's safe. Good that he did it, but it's 4 years late. He would have still won in 2008 because the people this statement turned off today were NEVER going to vote for him then or now.
Why are you so sure about that? Independents are stupid, fickle voters who decide the election. I don't think gay marriage would be the biggest issue to them, but it would just be another to influence them.
edit: And I mean this election. You're right about 08. He had such a big lead that it wouldn't have mattered.