This is an awesome event. DOMA is one of the major things standing in the way of full marriage rights in the US, though I don't think anyone is deluding themselves into thinking that such rights are likely to come from the legislature. Civil rights tend to be protected by the aloof courts first and the populist legislature last, which is of course why we have the courts. <3
Two caveats, though: firstly, the administration isn't exactly
saying they will not defend DOMA period end to statement. There are a few 'tests' of constitutionality that can be applied to laws which infringe upon the rights of cohesive groups. The first is 'rational basis,' which basically says 'look, if you can think of a good reason, I'm sure it's fine.' That's the easy one. The harder one in which we're interested is 'strict scrutiny,' which asks a number of questions, including whether the group is cohesive, has immutable attributes marking members as members, and has been disenfranchised and the subject of discrimination, as well as whether the law is specifically tailored
to a single, particular, extremely compelling governmental goal.
Up until now, the cases have been in court systems where precedent has held that issues of gay rights should be subject to the easy rational basis test, and the administration feels it can advance an actual argument under that test (if not a very compelling one; really, many right-wing groups accused the administration of 'throwing the trial' because there isn't a good argument even at that level
). However, recently, cases were brought in court systems without such precedent, and this required the administration to perform its own analysis. This analysis seems to have reached the conclusion that issues of gay rights should be considered using the much more difficult strict scrutiny test. As such, the administration will be using that level and urging the courts to use that level wherever possible, which means that where it's not bound by precedent it won't be defending the law at all. It may still continue to defend it in the court systems where precedent binds it, but those arguments are so crappy members of Congress may just step in to defend it anyway— apparently, Congress has the right to defend a law, and members may well step in as defendants-intervenor. The Justice Department foresaw this, though, and noted that Congress's only defense is the legislative record, which is shot through with Neanderthal comments and the sort of animus that could cause most people today to cringe. Congress can defend it, but its arguments are not very compelling, and may indeed work against them.
The other caveat (yay, wall of text) is that the cases being brought aren't against Section 2 of DOMA, which is the part that overrides full faith and credit and allows states to refuse to recognize same-sex marriages made under the authority of other states. They're being brought against Section 3, which is the part that refuses federal recognition to those marriages. It's probable that Section 3 will be deemed unconstitutional on at least the circuit and appeals levels, which will likely mean that marriages made in those districts will receive federal recognition eventually. Beyond that, a nationwide striking down would require the Supreme Court.