DOMA (Defense of Marriage Act), the existing law that forbids federal recognition of gay marriage in the United States, prohibits the recognition of same-sex couples under federal law. What this means is the usual tax breaks that are given to straight married couples are not offered to gay and lesbian couples, even if they are legally married on the state level. There's an inequality in the tax law. Essentially, gay and lesbian citizens are essentially penalized for not being straight.
Pres. Obama promised equality for LGBT Americans in this second inaugural speech this week. The news is promising and implies that he'll give his full support (translation political power) to helping rid unequal laws like DOMA from the books. It's too late to effect the tax laws this season, but it is possible that gay and lesbian couples everywhere could get a bigger refund soon.
Here's more info on tax laws for gay and lesbian couples.