Specifically, under a feature of California law that recognizes domestic partnerships gay couples must now combine their income and report half of it on each of their respective returns.
The ruling marks the first time that the IRS has recognized same-sex couples as equal to their heterosexual counterparts for tax purposes. Of the community-property states (i.e. all property and debt is owned equally by a couple) Nevada and Washington also recognize domestic partnerships, so couples there may also be affected.