The Supreme Court could make history-changing decisions on same-sex marriage as early as Monday. If the court decides NOT to take up two same-sex marriage cases, that will be the same as ruling in favor of gay marriage in each case.
First, they're considering the Defense of Marriage Act. If SCOTUS decides to hear any of the DOMA petitions, it will end up deciding whether DOMA's statement that marriage is "between one man and one woman" violates the equal protection clause.
If SCOTUS decides not to hear the petitions, then DOMA will be struck down, because two lower court rulings that overturned DOMA will stand. In that case, gay marriage will be federally recognized marriage in all of the states where it's currently legal.
Second, they're considering California's Prop. 8. If they decide not to hear it, that will make gay marriage legal in CA by default, because it will leave the lower court's ruling in place. If SCOTUS does decide to hear the Prop. 8 case, however, that will delay the ruling until early 2013.