For primary homes purchased before Dec 1, 2009, the max credit is $8,000 and it need not be paid back unless the home is sold within three years of purchase.
The 2009 purchase can be carried back to 2008 for the full $8,000.
Homes purchased in 2008 still have the 15-year repayment rule.
The credit is fully refundable.. meaning, to be received in full even if it exceeds the tax bill for the year.