http://www.irs.gov/businesses/article/0,,id=214841,00.html
Qualified Plug-in Electric Drive Motor Vehicles (IRC 30D)
...
For vehicles acquired after December 31, 2009, the credit is equal to $2,500 plus, for a vehicle which draws propulsion energy from a battery with at least 5 kilowatt hours of capacity, $417, plus an additional $417 for each kilowatt hour of battery capacity in excess of 5 kilowatt hours. The total amount of the credit allowed for a vehicle is limited to $7,500.
The credit begins to phase out for a manufacturer’s vehicles when at least 200,000 qualifying vehicles have been sold for use in the United States (determined on a cumulative basis for sales after December 31, 2009). For additional information see Notice 2009-89.
The qualified plug-in electric drive credit under IRC Section 30D no longer applies to low speed vehicles. However, the credit under section 30, discussed below, applies to certain low speed vehicles acquired after December 31, 2009.