Your basic question has been answered already: Because the tax credit is contingent on the purchase of a property (and your meeting various other conditions), you don't get the tax credit until you buy. So--with the exception of certain plans that will "advance" you the equivalent of the tax credit--no.
Regarding your question regarding contract for deed, two thoughts:
A contract for deed won't qualify for the tax credit unless you meet some other conditions. From the IRS web site at http://www.irs.gov/newsroom/article/0,,id=206291,00.html
Q. Can a taxpayer claim the first-time homebuyer credit if the purchase is pursuant to a seller financing arrangement (for example, a contract for deed, installment land sale contract, or long-term land contract), and the seller retains legal title to secure the taxpayer's payment obligations?
A. If the taxpayer obtains the "benefits and burdens" of ownership of a residence in a seller financing arrangement, then the taxpayer can claim the credit even though the seller retains legal title. Factors that indicate that a taxpayer has the benefits and burdens of ownership include: 1. the right of possession, 2. the right to obtain legal title upon full payment of the purchase price, 3. the right to construct improvements, 4. the obligation to pay property taxes, 5. the risk of loss, 6. the responsibility to insure the property and 7. the duty to maintain the property. (7/2/09)
So, for instance, to have a contract for deed qualify, you'd need to have the right to construct improvements; you'd need the obligation to pay property taxes; you'd need to be responsible for insuring the property; and you'd need to be responsible for maintaining the property. All of that's possible under a contract for deed, but you'd have to make sure that the documents covered those points.
Another point: If you're looking for contracts for deed, I recently wrote a blog on how to find lease-options and similar arrangements. See http://www.trulia.com/blog/don_tepper/2010/03/how_you_can_fi
Hope that helps.