I'm very surprised that people are treating Obama's public statement as a watershed moment. Obama has publicly supported gay marriage since 1996, and only in recent years has cravenly pretended that he hasn't. Even now, Obama couches his support in terms of "states' rights," a concept the administration has repeatedly rejected, most recently in suits against states attempting to protect against voter fraud. As a policy matter, Obama's "evolution" has no effect: the Department of Justice was already breaking with its obligation to enforce federal law and litigating against DOMA on grounds that would guarantee a constitutional place for gay marriage.
I suppose one could celebrate that gays are now sufficiently accepted within society that the Obama administration felt it was politically more dangerous to continue its disingenuous hypocrisy on the issue than to publicly come out with half-hearted support for gay marriage. (As a political calculation, the Democratic voting bloc most likely to oppose gay marriage—African-Americans—is unlikely to leave Obama, despite the disastrous consequences his policies are having for that community.) And the administration has now successfully distracted the news cycle from the poor economy for a full four days. But they will be hard pressed to complain about the fact that Mitt Romney flip-flopped on social issues to appease his base.