Bad news for you and I both the way I see it.
But I admit I haven't seen enough info yet to determine a "backdate", I don't think there is one.
1) The tax credit is really a postponement in the first place. For the $8,000 in existence, yes you get it this year, but must repay it at $1,600/year over the next five years, so it's really a tax postponement, an interest-free loan.
That much I do know
2) The new law reads to me that only newly created home purchase contracts are eligible. I don't see anything about backdating???
http://news.yahoo.com/s/ap/us_homebuyers_tax_credit
part of the text reads-
First-time homebuyers have been getting tax credits of up to $8,000 since January as part of the economic stimulus package enacted earlier this year. But with the program scheduled to expire at the end of November, the House voted 403-12 Thursday to extend and expand the tax credit to include many buyers who already own homes. The Senate approved the measure Wednesday, and the White House said President Barack Obama would sign it Friday.
Buyers who have owned their current homes at least five years would be eligible for tax credits of up to $6,500. First-time homebuyers — or anyone who hasn't owned a home in the last three years — would still get up to $8,000. To qualify, buyers in both groups have to sign a purchase agreement by April 30, 2010, and close by June 30.