The sweeping California law that granted registered domestic partners many of the rights of married couples is raising confusion over one of the building blocks of a tax return: Whose income is it? The law, which took effect in 2005, treats two basic types of income differently on California tax returns. Though all income is considered community property, the law specifically bars registered partners from splitting their combined paychecks and other earned income on their state tax returns. They do split unearned income, such as dividends from stock. Michael J. McIntyre, a law professor at Wayne State University in Detroit , predicts the courts would shoot down income-splitting. That said, he thinks there\'s a strong enough argument to press the case. If the IRS rejects income-splitting, it\'s unclear how partners should treat money acquired as community property. Is it income? A gift?