Fox News senior judicial analyst Andrew Napolitano wrongly asserted that the Obama administration's decision to not defend the federal Defense of Marriage Act, which denies same-sex couples more than 1,000 federal benefits and protections, is unprecedented, as previous administrations have also declined to defend statutes they considered unconstitutional.

On March 27, the same day the Supreme Court heard oral arguments in Windsor v. United States, the challenge to DOMA, Napolitano said on Fox's Happening Now:

The president has taken an oath to uphold the law.  All the laws, whether he agrees with them or not.  But he has forbidden the Justice Department from defending this law.

[...]

That's the question, because the government can't write a law for no reason. Every law has to have a rational basis. It has to have some reason. Any reason that makes sense. So the government would have to argue, here's the reason for the law. But the government is not in the courtroom. In fact, President Obama dispatched the government lawyers to argue against this law, which is truly unheard of in my experience.

In fact, there is precedent for refusing to defend a statute.  In a letter to Congress explaining the administration's position that DOMA is unconstitutional, Attorney General Eric Holder explained:

[T]he Department in the past has declined to defend statutes despite the availability of professionally responsible arguments, in part because the Department does not consider every plausible argument to be a “reasonable” one.   "[D]ifferent cases can raise very different issues with respect to statutes of doubtful constitutional validity," and thus there are “a variety of factors that bear on whether the Department will defend the constitutionality of a statute.”   Letter to Hon. Orrin G. Hatch from Assistant Attorney General Andrew Fois at 7 (Mar. 22, 1996).   This is the rare case where the proper course is to forgo the defense of this statute.   Moreover, the Department has declined to defend a statute “in cases in which it is manifest that the President has concluded that the statute is unconstitutional,” as is the case here.   Seth P. Waxman, Defending Congress, 79 N.C. L.Rev. 1073, 1083 (2001). 

In fact, the George W. Bush, Clinton, George H.W. Bush, and Reagan administrations all have declined to defend statutes they concluded were unconstitutional.