A Federal District Court judge has ruled Section 3 of the Defense of Marriage Act, which defines marriage as only between a man and a woman, is unconstitutional.
The relevant part of the ruling U.S. District Judge Joseph Tauro states:
The ruling seems to be based on the Tenth Amendment and the Due Process Clause of the Fifth Amendment. The basic idea being that the states have the power to regulate marriage, not the federal government.
Of course this is only the district court level so I expect there to be appeals to the Court of Appeals and eventually the US Supreme Court.
The ruling came from two cases: Gill v. OPM, brought by GLAD, and the Commonwealth of Massachusetts v. HHS, brought by Mass. Attorney General Martha Coakley.