A three judge panel of The United States Court of Appeals for the First Circuit just handed down a decision declaring the anti-gay Defense of Marriage Act unconstitutional. Notably, the panel included Judges Juan Torruella and Michael Boudin, both of whom are Republican appointees. Judge Boudin, who authored the opinion, is one of the most highly regarded judges in the country; he frequently sends his former law clerks to clerk for Supreme Court justices.
Here's the money quote from the ruling linked above (PDF, emphasis mine):
For 150 years, this desire to maintain tradition would alone have been justification enough for almost any statute. This judicial deference has a distinguished lineage, including suchfigures as Justice Holmes, the second Justice Harlan, and Judges Learned Hand and Henry Friendly. But Supreme Court decisions in the last fifty years call for closer scrutiny of government action touching upon minority group interests and of federal action in areas of traditional state concern.
To conclude, many Americans believe that marriage is the union of a man and a woman, and most Americans live in states where that is the law today. One virtue of federalism is that it permits this diversity of governance based on local choice, but this applies as well to the states that have chosen to legalize same-sex marriage. Under current Supreme Court authority, Congress' denial of federal benefits to same-sex couples lawfully married in Massachusetts has not been adequately supported by any permissible federal interest.
Both the "closer scrutiny" Fifth Amendment and the "permissible federal interest" Tenth Amendment arguments are key to taking down DOMA. The basic argument is that the federal government has a duty to protect minority groups under the Equal Protection clause, and that same-sex couples meet the standard of a minority group that needs equal protection, and as such the denial of federal marriage benefits specifically to same-sex couples violates that equal protection when the state, in this case Massachusetts, makes those benefits at the state level legal. It's the scope of how far that applies (if it's just to states where same-sex marriage is legal or all 50 states) that seems to be the question the panel is leaving up to SCOTUS to chew over, but they definitely accept the argument as valid.
The ruling also spells out pretty plainly that in the end, only SCOTUS can decide this one. But seeing the lower court ruling affirmed here is a definite point for the good guys.