Although the actual amount of the tax credit hasn't been confirmed by the IRS, it's widely expected to be as much as $3,150. The size of the credit depends on the government fuel efficiency rating, and the 60 mpg (city) Prius should qualify for the maximum tax break. Less fuel-efficient hybrids receive smaller tax credits - Ford's Escape Hybrid 4WD model qualifies for an estimated $1,050, for example.
For more info on these and other energy-related tax incentives, check out the Tax Incentives Assistance Project website. Estimated tax incentives for qualifying vehicles are available from the American Council for an Energy-Efficient Economy, here.