Interesting. Irrespective of what you think about DOMA or gay marriage, I think this seems like good tactics, but bad strategy (from a Democrat point of view, anyway).
To illustrate what I mean, does this now mean a future GOP administration could decide not to defend the Constitutionality of Obamacare and the individual mandates? I think it's problematic for the administration to set a precedent of not defending properly passed laws with which it disagrees - people complained about the Bush DOJ being politicized, but they defended McCain-Feingold, even while maintaining it was unconstitutional as an overall judgment.
On the politics of gay marriage, it seems to me that Obama is trying to have it both ways here. Holder specifically claims that this is consistent with DOJ's "longstanding practice of defending the constitutionality of duly-enacted statutes if reasonable arguments can be made in their defense." But, given the briefs filed in these cases already, it would seem there are reasonable arguments to be made (on both sides).
Lastly, Holder says that the Obama DOJ "will continue to enforce" DOMA. But it seems to create a paradox for the Obama administration to refuse to defend DOMA and to continue to enforce it. The most obvious reason I find to resolve the paradox is also cynical: Obama doesn't want to take political heat for failing to enforce DOMA, but he's hoping that his refusal to defend it will lead to court rulings in his favor (and he increases those odds if he refuses to defend the law). But in politics, cynicism is often the right instinct.