That’s the decision this afternoon, based on equal protection principles applied to the federal government through the Fifth Amendment’s Due Process Clause. Section 3 of the Defense of Marriage Act of 1996 established a federal definition of marriage for the first time. This meant that the federal government could refuse to grant validly married same-sex couples the federal benefits and privileges accorded opposite-sex married couples. I’m still looking at the decision and will probably post soon.
UPDATE: In a companion case, the same judge has ruled that DOMA intrudes on the Tenth Amendment powers of the states. Very interesting.