The IRS ruled that same-sex couples with domestic partnerships in California must put down the average of their incomes when they file federal income taxes, reducing federal income tax liability for same-sex couples where one person makes significantly more than the other. California is a community property state, so the IRS found that the best way to account for these people's incomes in accordance with California law was to consider half the income they make together as belonging to either person.
The Wall Street Journal explains how it works with the couple that asked for the rule clarification:
Applying this rule to federal taxes offers clear tax benefits for people such as Mr. Rey--an executive who said he earns much more than his partner does--because it brings him into a lower tax bracket. In 2007, he said, applying this standard would have cut his federal tax liability in half and more than doubled his partner's tax bill. Taken together, it would have saved them about $7,000, he said.
Nevada and Washington are also community property states that have domestic partnerships, and the WSJ says they may be affected by this ruling as well. Wisconsin's also a community property state with domestic partnerships, but I guess they don't apply community property to DP's.