@snood,
None of those presidents is running for reelection in 2012, so I don't see how that matters.
Obama's change of mind isn't big news about Obama, it's big(ish) news about American society and its changing norms. Obama has taken the safe position in
Audacity of Hope (2006), and he is taking the safe position now. What has changed between 2006 and 2012? Only the location of that safe position. And now that it has, Obama is catching up with it.