Sarah Farley had worked at a law firm where she participated in the firm's Profit Sharing Plan - a plan qualified under the Employee Retirement Income Security Act (ERISA). The Plan provides that death benefits be paid to the participant's "surviving Spouse." Sarah then married Jean Tobits in Canada. When Sarah died, both Jean and Sarah's parents claimed the death benefits.
The dispute went to federal district court in Pennsylvania (Cozen O’Connor PC v. Tobits) where the judge had no trouble deciding that Jean was Sarah's surviving spouse.
In United States v. Windsor (US Supreme Court 06/26/2013) the Supreme Court held that Section 3 of the Defense of Marriage Act (DOMA) - defining "spouse" as a person of the opposite sex - is unconstitutional. Therefore, since Sarah and Jean were lawfully married, and that marriage is recognized by the laws of Illinois, ERISA has to be interpreted as meaning Jean was Sarah's spouse. And thus the law firm's ERISA plan has to be interpreted as meaning Jean was Sarah's spouse.
This leaves me with one huge question: Will you get the same result in every state? That seems doubtful to me. The opinion in Windsor (a 5-4 decision) relied heavily on the fact that Windsor's same-sex marriage was recognized by the State of New York (and the Tobits marriage was recognized by the State of Illinois). As Justice Kennedy put it, "DOMA’s principal effect is to identify a subset of state-sanctioned marriages and make them unequal." So, if you're in a state where same-sex marriages are not recognized, it may be difficult to apply the logic of the Windsor case.