It's not as big as desegregation under Wallace, by a long shot, which was the last time the federal executive stepped in; this makes Alabama, what, the 37th state in which same-sex marriage is legal? The tide has turned on this one.
Since it now seems to be a given that the Supreme Court will rule in favor of equal rights early this summer, my guess is that the Obama administration wouldn't act until that happens, if Alabama (or other states) chose to defy the ruling after that.
In the meantime, there's that ethics complaint. There's also the possibility that Judge Callie Granade, the federal district court judge who just ruled Alabama's law unconstitutional, could find Justice Moore in contempt. I suppose she could even throw him in jail -- wouldn't that be delicious?