There's a great side-by-side chart located at http://docs.Steven-Anthony.com/033010ComboTaxCreditChart.pdf that breaks down all the details of both the FED & CA Tax credits.
For example, the FED requires you maintain the property as your primary residence for three years. The State requires 2 years. With the FED tax credit you claim it all in your 2010 taxes. The State tax credit is claimed over the 2010-2012 tax years.
There are many other comparisons on the chart.
Regarding paying it back, from the IRS Web site:
A. If, within 36 months of the date of purchase, the property is no longer used as your principal residence, you are required to repay the credit. Repayment of the full amount of the credit is due at the time the income tax return for the year the home ceased to be your principal residence is due. The full amount of the credit is reflected as additional tax on that year's tax return. See Form 5405 and its instructions about repayment of the credit. (5/6/09, 1/27/10)
For more information, see http://www.irs.gov/newsroom/article/0,,id=206291,00.html