Aside from the Big Deal that DOJ no longer will defend a federal law (which historically has hardly ever happened) there's perhaps an even Bigger Deal in today's announcement that President Obama has decided the Defense of Marriage Act is unconstitutional.
In reaching that conclusion, the President and Justice Department have concluded that laws treating gays and straight people differently must get closer scrutiny from the courts--and should be upheld only if there is a very good reason for them. That's a tougher legal standard than the courts typically have used in evaluating discriminatory laws against gays and lesbians.
It also puts the Administration in front of the Supreme Court, which has yet to decide whether gays and lesbians should get the same kind of protection that women or, even, that minorities get. If the Court ultimately agrees with the Justice Department on using a tougher standard, in practice that means pretty much any law treating gays and lesbians differently than everyone else will almost always be unconstitutional.
Dale Carpenter, writing at Volokh, has some interesting perspective:
"The Obama administration has now formally put the weight and authority of the Executive Branch behind the proposition that government discrimination against gays and lesbians is unconstitutional under the equal protection principles of the Fifth and Fourteenth Amendments. Since marriage between two people of the same sex, whether unrecognized by the federal government under DOMA or barred by the states under their own marriage laws, discriminates against gays and lesbians and cannot survive heightened scrutiny, it follows that the laws of 45 states barring gay marriage are unconstitutional. While the DOJ won't formally be attacking state marriage laws in federal courts, its views do carry some persuasive heft."