Off of the IRS website:
Q. If I claim the first-time homebuyer credit for a purchase in 2009 or early 2010 and stop using the property as my principal residence before the 36 month period expires after I purchase, how is the credit repaid and how long would I have to repay it?
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. Form 5405 and its instructions will be revised for tax year 2009 to include information about repayment of the credit.