In response to the senior tax advisor from HR Block:
"If I’m qualified for the tax credit and buy a home in 2009, can I apply the tax credit against my 2008 tax return?
Yes. The law allows taxpayers to choose ("elect") to treat qualified home purchases in 2009 as if the purchase occurred on December 31, 2008. This means that the 2008 income limit (MAGI) applies and the election accelerates when the credit can be claimed (tax filing for 2008 returns instead of for 2009 returns). A benefit of this election is that a home buyer in 2009 will know their 2008 MAGI with certainty, thereby helping the buyer know whether the income limit will reduce their credit amount.
Taxpayers buying a home who wish to claim it on their 2008 tax return, but who have already submitted their 2008 return to the IRS, may file an amended 2008 return claiming the tax credit. You should consult with a tax professional to determine how to arrange this. "
I know that this applied for the $7500 and my understanding is that it is applicable for the $8,000 credit as well.
I assume it will take a while for them to get the new form posted but if you call the IRS' helpline they are very helpful and they may just tell you how you can claim the new credit with the current form.