Before my gay-rights-loving friends get too excited, here's a very important part of Eric Holder's letter to Congress:
The President has also concluded that Section 3 of DOMA, as applied to legally married same-sex couples, fails to meet that standard and is therefore unconstitutional. Given that conclusion, the President has instructed the Department not to defend the statute in Windsor and Pedersen, now pending in the Southern District of New York and the District of Connecticut. I concur in this determination.
Notwithstanding this determination, the President has informed me that Section 3 will continue to be enforced by the Executive Branch. To that end, the President has instructed Executive agencies to continue to comply with Section 3 of DOMA, consistent with the Executive’s obligation to take care that the laws be faithfully executed, unless and until Congress repeals Section 3 or the judicial branch renders a definitive verdict against the law’s constitutionality. This course of action respects the actions of the prior Congress that enacted DOMA, and it recognizes the judiciary as the final arbiter of the constitutional claims raised.
I'm ... not so impressed by this. "It's unconstitutional, but we'll enforce it" is ... lousy. Possibly even indefensible. I'm not certain what the federal government actually does to enforce the law, so it might be a moot point, but it's possible the president is making a very loud noise over very little substance here.