According to the IRS press release, the amount of the tax credit is determined by the "date of purchase." The "placed in service date" determines in what tax year you claim the credit. The example they give is buying a Prius on Sept. 30, 2006 (the day before the full $3,150 tax credit was cut in half) but not "taking possession" (their words) of the car until Feburary 2007. The buyer, according to the IRS, would be entitled to the full tax credit, but would have to claim it for the 2007 tax year (the filing deadline for which was April 15, 2008). What the IRS doesn't really explain is what they mean by "purchase." I would think that the signing of the bill of sale and the funding of the purchase (either with cash or with a loan) would qualify. But I'm not a tax lawyer, or an accountant, or a bureaucrat . . .