Personally, I think this helps Obama greatly in more liberal and democratic states but will hurt him in the "Bible Belt" South. And he has all but certain lost North Carolina. I find this backing of marriage equality nice, but I am skeptical of anything actually changing. I tend not to trust politicians during election years.
So, more questions to be answered I assume:
How will this affect his campaign?
How is this being received in the gay and lesbian community?
And maybe the most important question of all, will anything actually change or will this be once again placed on the back burner after the election (assuming Obama is reelected)?