I don't think that the IRS can be made to return the estate taxes collected from same-sex widow(er)s because it was perfectly legal to collect them at the time, and retroactive application of the law is dicey at best. It might not have been right, but it was legal.
Law isn't my strongest area of politics, so I could be wrong here, but I don't see why not. The law wasn't repealed, it was struck down as being unconstitutional, and therefore illegitimate. I was under the impression that Edith Windsor, the original plaintiff in the DOMA case, was suing to get back the $363,000 in estate taxes that she wouldn't have had to pay if her partner had been a man. If I understand correctly, the court has agreed that the law was discriminatory, and she shouldn't have had to pay, implying that the money will be reimbursed. If I'm right, other gay couples who have been negatively financially affected would have to bring suit to request that their money be reimbursed.
Can anybody with a law degree verify/invalidate this?