If DOMA is unconstitutional, what kind of past employee benefits will need to be paid?

Don't assume the effect of a new decision will be only prospective. Consider this report of a ruling by the Judicial Council of the Ninth U.S. Circuit Court of Appeals:

... Christopher Nathan, 39, of San Francisco, a law clerk for U.S. Magistrate Maria Elena James, sought [health insurance] coverage for his spouse, Thomas Alexander, 40.... [H]e was turned down by the Administrative Office of the U.S. Courts because the 1996 law bars federal recognition of same-sex unions.

In April, Chief U.S. District Judge James Ware said the denial violated the federal court's rules against discrimination based on sexual orientation and gender, and ordered the court to reimburse Nathan for the costs of buying private insurance.

The Judicial Council, the final authority in the administrative review process, went a step further in this week's order and said DOMA has been held unconstitutional by a San Francisco federal judge in another employee's case. The three-judge panel ordered the court [that is, his employer] to determine how much it owes Nathan and then pay him within 10 days.