If you have plans to buy your first home, you better hurry. The first-time homebuyer credit expires November 30, 2009. There is no plan to extend the credit at this time.
The $8,000 credit is refundable, meaning, if your taxes are under $8,000, the excess will be paid to you. You can amend your 2008 tax return to claim the credit immediately.
The old credit for 2008, capped at $7,500 was no more than an interest-free loan. Those credits must be paid back in 15 equal installments starting with the 2010 tax return.
For 2009, the credit expanded to $8,000 and doesn't get paid back. The home must remain your primary residence for three years or the credit is forfeited.
You must close on the home prior to filing an amended return for the credit. Use form 5405. The IRS is slow sending checks for this credit due to the volume received and for fraud prevention. The credit has turned into a fraud problem for IRS, so they are reviewing all claims for the credit.
Follow-up note: After I wrote this post I read that there is an effort in Congress to extend the credit. Stay tuned; I'll fill you in if it is extended.