First of all, the Administration defended DOMA for two years, and Eric Holder's explanation of the change is weak at best (one challenge in the Second Circuit changes the DOJ's entire stance on the issue?). In an interview with Salon.com, constitutional law professor Jonathan Turley says, "It's not very persuasive to say that the administration has suddenly concluded that DOMA is unconstitutional. The more obvious explanation is that it didn't feel it could politically oppose DOMA before the midterm elections. I found Holder's statement to be rather forced and unconvincing."
I might be more convinced if the DOJ were not planning to "remain party to the cases and continue to represent the interests of the United States throughout the litigation" of two pending cases in which they were previously defending DOMA.
Second, this isn't the first time the DOJ has announced that they will no longer defend a law with which they disagree. Eric Holder announced more than once that the Department of Justice would not target marijuana use in states whose laws conflicted with federal laws. In spite of these claims, the DOJ never stopped enforcing federal drug laws . We've seen this show before.
So they aren't repealing DOMA. They are just refusing to defend it. My guess is that, like it did with federal drug laws, the Administration will quietly continue the previous policy, while hoping that this announcement is enough to please the necessary voting blocs. Sounds more like weak capitulation than an actual change in policy.